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biged85

Seating Depth Rule of Thumb

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Hi All,

I was wandering how many of you subscribe to the idea that when seating a bullet it is wise to have one bullet diameter of bearing surface in the case neck? So for example, with my Creedmoor I should have .264"

 

The reason I ask is my Bergara BMP has a very long throat and I am trying to advance my reloading skills by experimenting with seating depths. I am using 123gr SST's and N150 @ around 2850fps producing .7 inch groups (COL 2.810) I would like to improve these groups as I have achieved 0.25" groups with 123gr Scenars so I know the rifle and myself are capable.

 

COL wise I can physically go out to 2.850 before the feed ramp interferes with mag feed (which I need to keep) and I am still miles from the lands at this point. What is the minimum amount of bearing surface you would use while attempting to get as closer to the lands?

 

Hope that all makes sense. 

 

Ed 

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This is a problem with many factory rifles - which are throated 'long' to cope with any situation/bullet.

The 'one calibre' seating depth is a good 'rule of thumb' for those just learning to reload. You can go much less than this - but not if you're ramming the bullet against the feed-ramp.  Load 'em carefully, by hand.

Having said all that - if you're getting 0.7 inch groups with a Bergara, I would say you've already exceeded the manufacturer's expectations.

 

 

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2 minutes ago, The Gun Pimp said:

This is a problem with many factory rifles - which are throated 'long' to cope with any situation/bullet.

The 'one calibre' seating depth is a good 'rule of thumb' for those just learning to reload. You can go much less than this - but not if you're ramming the bullet against the feed-ramp.  Load 'em carefully, by hand.

Having said all that - if you're getting 0.7 inch groups with a Bergara, I would say you've already exceeded the manufacturer's expectations.

 

 

HI gun pimp,

I don't think i will ever buy an off the shelf rifle again, I would much rather get something re-barrelled to better specs. It makes no sense to me about the really long throat and then to compound things the feed ramp gets in the way meaning you cant even load to mag length. Though apparently some people get gun smiths to alter this.

The rifle shoots very well indeed but I have a feeling it could of been even better which is a little frustrating.

 

ATB

 

Ed

 

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The T3x TAc A1 allows rounds like the 139 Scenar to be loaded within 20 thou off the lands and still within mag length.  I wish more manufacturers would follow suit. As above, 0.7 is respectable if it's a 10 shot group.  Have you tried experimenting with primers or tried OCW as a method of load development to see where your min ES lies as that is another way to experiment?

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I've regularly used half-calibre shank depths in the neck over the years, on many occasions  down to a tenth of an inch, and with good results in calibres from 6mm up to .30.  That's been with single-loading and careful transport and handling though. How well these rounds would stand up to magazine loading and feed .............. ? 

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I have ran as little as .050" of bullet in the neck.  I think it will depend on your loads requirement for neck tension.  Some combos need a good amount of neck tension for consistent accuracy.  That load I shot was with Varget, which has always responded well to very light neck tension for me.

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21 hours ago, biged85 said:

Hi All,

I was wandering how many of you subscribe to the idea that when seating a bullet it is wise to have one bullet diameter of bearing surface in the case neck? So for example, with my Creedmoor I should have .264"

 

The reason I ask is my Bergara BMP has a very long throat and I am trying to advance my reloading skills by experimenting with seating depths. I am using 123gr SST's and N150 @ around 2850fps producing .7 inch groups (COL 2.810) I would like to improve these groups as I have achieved 0.25" groups with 123gr Scenars so I know the rifle and myself are capable.

 

COL wise I can physically go out to 2.850 before the feed ramp interferes with mag feed (which I need to keep) and I am still miles from the lands at this point. What is the minimum amount of bearing surface you would use while attempting to get as closer to the lands?

 

Hope that all makes sense. 

 

Ed 

You seem fixed on the idea you need to advance forward towards the lands to find better accuracy? 

If you look for them there will be seating depth accuracy nodes at different ranges of seating depth, if its not good at mag length then start working backwards but be mindful of pressure, if your running close to a case full you might be best to starting a bit lower on the charge weight. Having the bullet seated deep will reduce your effective case capacity but if you find accuracy a little slower than you would want its not the end of the world, just start 1.0-1.5gr lower with your charge weights.

Jumping a bullet 0.120" or more isn't the end of the world if its accurate and safe. Try some seating depth tests at mag length then go shorter in 0.040" increments. There will be a stand out length and then fine tune around that area. Dont worry if your jumping 0.160" if your throat allows it and the bullet isn't sat too deep in the case, I once had a load that was jumping 0.160" due to similar circumstances and it shot very well indeed.

Ideally you would have the freebore optimised to suit the range of bullets you plan to shoot if you were going the rebarrel route but just because you have a long freebore on the Bergara doesn't mean you can't find the accuracy you are looking for, you just need to know where and how to look for it.

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I would agree with Big Al.

I have owned and used a good few 6.5x55 rifles that had very long throats and got them shooting well doing what Al says. The most accurate bullet I used in my last 6.5 was the 85 grain Sierra and it was miles from the lands. 

 

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1 hour ago, Big Al said:

You seem fixed on the idea you need to advance forward towards the lands to find better accuracy? 

If you look for them there will be seating depth accuracy nodes at different ranges of seating depth, if its not good at mag length then start working backwards but be mindful of pressure, if your running close to a case full you might be best to starting a bit lower on the charge weight. Having the bullet seated deep will reduce your effective case capacity but if you find accuracy a little slower than you would want its not the end of the world, just start 1.0-1.5gr lower with your charge weights.

Jumping a bullet 0.120" or more isn't the end of the world if its accurate and safe. Try some seating depth tests at mag length then go shorter in 0.040" increments. There will be a stand out length and then fine tune around that area. Dont worry if your jumping 0.160" if your throat allows it and the bullet isn't sat too deep in the case, I once had a load that was jumping 0.160" due to similar circumstances and it shot very well indeed.

Ideally you would have the freebore optimised to suit the range of bullets you plan to shoot if you were going the rebarrel route but just because you have a long freebore on the Bergara doesn't mean you can't find the accuracy you are looking for, you just need to know where and how to look for it.

Firstly, thank you all for your replies.

I have done a OCW test and have had good results with a ES of 17fps which I am hoping to improve on with some basic work on the brass. I shall have a play with seating depth in both directions and see what I come up with, being cautious of pressure as I go.

When the weather clears up a bit I shall report back my findings :) I am secretly hoping to find a decent sweet spot........ 

Many Thanks

 

Ed

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I'm a bit of a country bumpkin but when there is no recommendation otherwise, I seat bullets with the parallel sides of the bullet seated to the base of the neck. (Neck shoulder junction) The boat tail is below that point. I am always surprised at how seldom I need to move from that setting. If i do it's usually a magazine fit issue.~Andrew

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Bullet seating depth is discussed almost as much as barrel cleaning and, the old carrot, barrel break-in.

I have to confess I don’t understand why (other than usable case capacity) bullet depth makes a difference to accuracy. 

I am lead to believe that it is all to do with optimal barrel time (OBT) and it’s relationship with barrel harmonics. Modelling this with OBT software and QuickLoad predictions bear this out to some degree. But, as those of us who use QuickLoad have discovered, some of the QL predictions are somewhat erroneous and can’t always be relied on.

Also, like a many others, I’ve found that secant ogive bullets much more sensitive to seating depth than tangent ogive bullets. This in itself would suggest that it’s nothing to do with OBT.

It would be interesting to hear a definitive answer to this.

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So would I Mark as my experience is similar to yours.  Whatever the answer, I normally load to magazine and play with charge weight for most bullets, rarely seating closer than 30 thou off to give some leeway.   I only keep secant ogive pills close (30 or less) to the lands, as like you, I've found many to be jump sensitive.

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3 hours ago, Brillo said:

Bullet seating depth is discussed almost as much as barrel cleaning and, the old carrot, barrel break-in.

I have to confess I don’t understand why (other than usable case capacity) bullet depth makes a difference to accuracy. 

I am lead to believe that it is all to do with optimal barrel time (OBT) and it’s relationship with barrel harmonics. Modelling this with OBT software and QuickLoad predictions bear this out to some degree. But, as those of us who use QuickLoad have discovered, some of the QL predictions are somewhat erroneous and can’t always be relied on.

Also, like a many others, I’ve found that secant ogive bullets much more sensitive to seating depth than tangent ogive bullets. This in itself would suggest that it’s nothing to do with OBT.

It would be interesting to hear a definitive answer to this.

There are lots of things accuracy related that we would all like definitive answers on but as you know they are rare and few people have tested enough to be certain.

That said I have yet to find a bullet regardless of ogive type that isn't affected by changes to seating depth. How much and the extent of the effect will depend on the extent of your testing, the level of accuracy your shooting system can hold as well as the distance you are testing over.

At 100yds its sometimes hard to see what a 5 thou change can do to group shape and size, take it out to 600 or 1000yds and if your testing platform is sound enough it will show up very clearly.

I would say the OBT concept is at least a good enough model to allow a load developer an understanding that seating depth is an independent variable to powder charge and something that should always be tested and optimised.

To call a load good without doing a seating depth test is to do half a load development in my opinion.

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I doubt if changing seating depth (or more accurately jump) has much to with OBT with the possible exception of very 'hard' seating into the lands that raises shot start pressure (and hence maximum  chamber pressure) a great deal. One presumes it will change barrel harmonics significantly.

Mostly it has to do with bullet alignment in the bore and how well or badly a bullet design self-aligns. Tangent ogive types are very good at aligning themselves with the bore, aggressive secant types are very poor. If the bullet enters the rifling misaligned, it will stay that way down the barrel and on exiting suffer significant precession or coning in flight. That's why most VLDs shoot well with a degree of seating into the lands - they are being thereby mechanically aligned with and by the bore.

What I can't explain is why such bullets sometimes do much better with very large jumps. One can only assume that since increasing jump sees the bullet arrive at the 'leade' travelling faster, then varying the amount of jump and using significant amounts of it on occasion produces an optimum entry speed that somehow improves alignment both in its degree and in consistency between shots. With true tangent ogive bullets, they align themselves on entry so well, entry speed apparently doesn't matter, or at any rate not very much.

The waters have been much muddied by recently introduced bullet designs with a neither fish nor foul situation appearing and extreme versions in others. The severity of the 'aggressiveness' of the nose to shank transition is described numerically by the Rt/R  metric. 1.0 is a perfect tangent / seamless junction; 0.5 is the value of the older VLD designs. The older Sierra MK designs that are regarded as very jump tolerant have values on or very close to 0.9; the 0.308" 175gn SMK specifically designed for the US forces 7.62 sniper round and adopted in the M118LR is one of those rarities that is actually a full 1.0 - and I know from experience  that changing seating depth / jump with it has no discernible effect on group dispersion. (It was designed for military use of course which means huge variations in barrel round counts and rifling leade erosion between different examples in service.) The 155gn 0.308 Berger VLD's Rt/R is 0.46 very close to the 0.5 definition and bullets like this are often 'b*gg*rs'  to tune. The very jump tolerant 155.5gn 0.308 Berger BT Fullbore  falls between at 0.76 and appears to be little if any affected by jump. I use the same load today at the same COAL with this bullet after nearly 3,000 rounds through the barrel and have never needed to 'chase the lands'. The groups are unchanged from when the barrel and load were new, but average MV has fallen by 23 fps no doubt due to the longer jump the bullet is making. And how do I know precision is as close to unchanged as I can measure? This rifle was used for my SR primer tests and that load was the 'control' in trying different primer makes for group size.  Also at various times in its life, it has been taken to a 100 yard BR range and three or four 5-round groups shot off the bench to check that all is 'still well'. The very tolerant 185gn Berger Juggernaut in the same calibre also has an Rt/R in this region, 0.72 according to Litz.

But we now have 'Hybrid' designs with no quoted Rt/R values as they don't lend themselves to this measurement. In theory the key bit of the bullet shape that determines alignment ability is as close to a perfect tangent as you can get so they will be COAL / jump insensitive. In my limited experience of the genre, a small number of 308s, they are anything but and every weight seems to need different treatment.

Now ... here's the interesting part! Litz / Berger introduces the former's wonderful 200gn .308 Hybrid design in c. 2011/2012, but 'everybody' ignores it at the FCWCs in Raton in 2013 and 'everybody' (bar us in the GB team) load 185gn Juggernauts instead. (We use 155s and pay a price thereby!) Four years later, Litz / Berger produce the 200.20X bullet, a new 200gn design and the US FTR team trials pre-production examples extensively in the run-up to the 2017 FCWCs at Ottawa. 'Everybody' (bar the Brits again who again use 155s and pay yet more dearly for that) uses this bullet and it is obviously the winner's choice and is apparently unbeatable. Now, with the Hybrid design dominating Berger's Target bullet listings, I had assumed the 200.20X was a mildly tweaked version of the Hybrid, but on getting hold of a copy of Litz's magnum opus Ballistic Performance of Rifle Bullets Volume 3, hey presto it's a completely different non-Hybrid model - a long-nose tangent ogive as per the 155.5 and 185 Juggernaut with an Rt/R value of 0.97 - way up and back with the good old Sierra MKs, almost with the venerable 175gn Sierra. ............. AND it has a higher BC than the same weight Hybrid!!

(Meanwhile in the relentless pursuit of BC to be competitive in a BC addicted market, Sierra is pumping out new designs that are ever longer, ever more secant, and have ever more aggressive nose-shank junctions. The 195gn 0.308 TMK is 0.46; the 183gn 7mm SMK has an exceptionally low 0.37.)

The importance of alignment is shown very clearly with long cast lead alloy rifle bullets which are much more fragile than jacketed types. Many successful models have a two diameter design, the front part being a long bore riding section that is only a little above the barrel  land diameter and whose purpose is to align the bullet really square in the bore through being marginally engraved by the rifling lands. The rear half of the bullet is a significantly larger diameter driving section that in the case of lead bullets will be larger than the barrel groove diameter, this making them 'fatter' here than normal jacketed bullet types.  Where the front section is undersize in relation to the barrel and doesn't do the alignment job, they rarely shoot well

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3 hours ago, Laurie said:

I doubt if changing seating depth (or more accurately jump) has much to with OBT with the possible exception of very 'hard' seating into the lands that raises shot start pressure (and hence maximum  chamber pressure) a great deal. One presumes it will change barrel harmonics significantly.

Mostly it has to do with bullet alignment in the bore and how well or badly a bullet design self-aligns. Tangent ogive types are very good at aligning themselves with the bore, aggressive secant types are very poor. If the bullet enters the rifling misaligned, it will stay that way down the barrel and on exiting suffer significant precession or coning in flight. That's why most VLDs shoot well with a degree of seating into the lands - they are being thereby mechanically aligned with and by the bore.

What I can't explain is why such bullets sometimes do much better with very large jumps. One can only assume that since increasing jump sees the bullet arrive at the 'leade' travelling faster, then varying the amount of jump and using significant amounts of it on occasion produces an optimum entry speed that somehow improves alignment both in its degree and in consistency between shots. With true tangent ogive bullets, they align themselves on entry so well, entry speed apparently doesn't matter, or at any rate not very much.

 

Thats an interesting theory Laurie, do you know of any hard test data that supports it?

My feelings are that sometimes I dont need to know the reason why something happens, its far more important to me to just acknowledge that it does and then understand the proccess that allows me to make the change and benefit from the results.

Through testing I can see the direct benefit of different powder charges in terms of stable POIs and/or vertical trends at distance and also with seating depth or jump changes (albeit we aren't always jumping) it shows at the target as good, bad or ugly - all of this is useful to me without actually understanding the physics, if anyone really does?

I remember watching Brian Litz talking about the benefit of Berger Hybrid ogives and the fact he said they were seating depth insensitive compared to other ogives yet this flew and still flies in direct contradiction to my own findings. Ive shot plenty of Hybrids since and seen far too many times how small seating depth changes affect group shape and size at both long and short range, so much so I now scratch my head at what he said.

As for bullet alignment, I agree entirely with the concept of introducing the bullet to the lands as concentrically as possible but for me that has to be done by accurate machining of the chamber in relation to the throat (and muzzle) and to its own concentricity as well as making sure the ammunition is as concentric as possible. From an engineering point of view I struggle to buy into the idea that by jumping a bullet more or less it will somehow more accurately align itself better but Im always open to reading any data thats out there.

When I mentioned OBT I should have made it clear I wasn't taking about Chris Longs theory per se but more that as we adjust the seating depth we alter the distance the bullet has to travel along the barrel before it exits, I call this barrel timing. We accept that barrel harmonics need to be timed favourably for bullet exit which powder charge or a tuner allows us to do. I then think that timing the bullet exit point even finer by seating depth is the final stage in the process, again if this is really whats happening is largely irrelevant so long as the results give me small groups at long distances. :)

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14 hours ago, Big Al said:

Thats an interesting theory Laurie, do you know of any hard test data that supports it?

My feelings are that sometimes I dont need to know the reason why something happens, its far more important to me to just acknowledge that it does and then understand the proccess that allows me to make the change and benefit from the results.

Through testing I can see the direct benefit of different powder charges in terms of stable POIs and/or vertical trends at distance and also with seating depth or jump changes (albeit we aren't always jumping) it shows at the target as good, bad or ugly - all of this is useful to me without actually understanding the physics, if anyone really does?

I remember watching Brian Litz talking about the benefit of Berger Hybrid ogives and the fact he said they were seating depth insensitive compared to other ogives yet this flew and still flies in direct contradiction to my own findings. Ive shot plenty of Hybrids since and seen far too many times how small seating depth changes affect group shape and size at both long and short range, so much so I now scratch my head at what he said.

As for bullet alignment, I agree entirely with the concept of introducing the bullet to the lands as concentrically as possible but for me that has to be done by accurate machining of the chamber in relation to the throat (and muzzle) and to its own concentricity as well as making sure the ammunition is as concentric as possible. From an engineering point of view I struggle to buy into the idea that by jumping a bullet more or less it will somehow more accurately align itself better but Im always open to reading any data thats out there.

When I mentioned OBT I should have made it clear I wasn't taking about Chris Longs theory per se but more that as we adjust the seating depth we alter the distance the bullet has to travel along the barrel before it exits, I call this barrel timing. We accept that barrel harmonics need to be timed favourably for bullet exit which powder charge or a tuner allows us to do. I then think that timing the bullet exit point even finer by seating depth is the final stage in the process, again if this is really whats happening is largely irrelevant so long as the results give me small groups at long distances. :)

Is not an alternative view on seating depth Al that increasing depth lowers pressures slightly, so has a similar effect as varying charge weight?

I have tried an experiment in the past of comparing groups shot using 69 TMKs at a fixed seating depth (15 thou off) varying the charge weight,  with increasing jump and keeping the same charge weight.  I did this initially as I found it hard to get this bullet to perform well, despite it being designed for the AR15 platform as being jump tolerant.

 In each case, you could see a pattern of groups opening up from optimum before shrinking back, such that there were about two nodes in each case, consistent with a barrel harmonic node being matched, in fact what you'd expect based on OBT charts for each charge weight overall bracket (ie max to max minus 10%).    I have no proof other than hypothesising that this was entirely consistent with pressure, hence velocity changes in each case affecting barrel time, and less to do with the distance that the bullet travels, since I don't buy such small changes in overall distance unduly affecting a node. 

I don't think that this approach would work using low Rt/R designs such as the new 30cal Sierra 155gr Palma (#2156) which seems to need to be loaded close to the lands.

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I,m happy with results from ensuring that as far as possible I,m building concentric ammo laying close to lands in a well smithed rifle build.To date I have never had a build not perform after arriving at the charge weight or weights that is best suited to all components.In some comments regarding finding optimum seating depth are we not just arriving at the optimum charge weight for a given seating depth in reverse order.Probably not explaining myself well but as one is discovering various seating depths have there own optimum charge weights and this bears with what had been reported from the many experiences written about in the "Secrets of the Houston Warehouse"you just have to take notice?......

One mad thought?,,,,do you think that moving the seating depth back gives chance for some sort of high pressure gas bearing that overtakes the bullet and centers up the bullet on entry to the lands?,,,,,,,sorry snowed in and nothing much else to think about,,,ha!,,,,O

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27 minutes ago, onehole said:

One mad thought?,,,,do you think that moving the seating depth back gives chance for some sort of high pressure gas bearing that overtakes the bullet and centers up the bullet on entry to the lands?,,,,,,,sorry snowed in and nothing much else to think about,,,ha!,,,,O

That has been postulated in the past and is by no means a silly thought. Whether it happens or not ........... ?

This whole issue of the interaction of bullet design and changing jump values is an immensely complex subject and the answer to Alan's question appears to be nobody knows exactly what mechanisms are, only that some do or don't work in individual precision rifles.

When I have time later today, I'll post some thoughts (for what they're worth which is probably b*gg*r all) and some fascinating links to discussion on some aspects of this. However, as a 'taster' here are some words by Bryan Litz from another forum written back in 2009. The source is a discussion on jumping VLDs large amounts which at the time was regarded as a novel if not near heretical view by many competition shooters.

 

We've got plenty of evidence just in this thread that secant ogive bullets can achieve their best accuracy with long jumps; something previously thought to be impossible or unlikely by many.

 

So we've learned something new. Some of us learned it sooner than others, but the point is that this subject of seating depth is not well understood. Sure we know that seating depth is a knob we can tune to refine the precision capabilities of our shooting sticks.

 

But does anyone know why seating depth affects precision?

 

Does anyone really know how seating depth affects precision?

 

I would love to know!

 

Here are a few facts that can be stated about seating depth and precision:

 

1. The precision of tangent ogive bullets tends to be less sensitive to seating depth than secant ogive bullets.

 

2. Internal case capacity is affected when seating depth varies.

 

3. The initial pressure spike on ignition is higher if the bullet is seated into the riflings compared to off the riflings.

 

4. The best seating depth for one rifle isn't always the best seating depth for another rifle, even when the rifles are very similar in construction.

 

If we could establish a cause/effect relationship between the variables and the result, imagine how huge that would be.

 

So who has some more facts to add to the list above? Anyone disagree with any of the existing 4? I'd like to change the direction of this thread from "It's possible to jump VLDs" to a more meaningful inspection of seating depth and precision in general. If we put our heads together, we just might come up with something and advance our collective knowledge of this mysterious tuning knob.

 

-Bryan

 

I thinks it's accurate to say that nobody has answered those questions in the intervening 9 years, but as I hinted previously, bullet design has moved on a lot with far more ultra long nose bullet designs and the Berger Hybrid type, but I'm not 100% convinced always for the better.

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13 hours ago, Big Al said:

As for bullet alignment, I agree entirely with the concept of introducing the bullet to the lands as concentrically as possible but for me that has to be done by accurate machining of the chamber in relation to the throat (and muzzle) and to its own concentricity as well as making sure the ammunition is as concentric as possible. From an engineering point of view I struggle to buy into the idea that by jumping a bullet more or less it will somehow more accurately align itself better but Im always open to reading any data thats out there.

 

I agree 100% with the importance / value of chamber concentricity and optimum throat dimensions and suchlike, but that doesn't necessarily mean that this aspect covers 100% of the issue.

The two-diameter lead bullet designs I mention are I imagine partly about getting this support / alignment issue right for a much more fragile and sensitive bullet type, but also because many such cast bullets are shot in mass produced bolt-action military rifles of the WW1 to WW2 era. The self-aligning bullet behaviour presumably ameliorates (overcomes?) the inevitable misalignments present in many such historic pieces.

Two-diameter jacketed bullets have occasionally been used, but not for self alignment reasons AFAIK. IIRC the 1960s 264 Winchester Magnum - a very hot, over-bore capacity design - used stepped bullets and Speer made a bullet specifically for handloaders of this number. The reason here was to reduce the length of the barrel shank / bearing surface and achieve higher velocities before exceeding the 65,000 psi maximum chamber pressure. (It didn't stop people allegedly burning out the barrel of their new Winchester Model 70  'Westerner' rifle in a single range session by firing too many rounds too quickly without breaks to cool the barrel. I wonder why this cartridge didn't survive long? :rolleyes:

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Thanks Laurie,,,Mmm bullet design,,,,don,t want to sidetrack this topic,,,,,will raise elsewhere if necessary,,,please advise...........something I noticed the other day,,,,,My 8 twist 22PPC,,,,,,,,,had two bullets performing really well 69TMK and 75Amax,,cloverleaf touching in fairly calm conditions,,,,went to range the other day{100m} ,,,,increased headwind wind 45 degrees left to right but not enough that I would have thought would have disturbed these slippery two!! but really disappointed,,,nothing I could do to keep tight at all,,,,interestingly I had brought along four different loads with 60g Berger varmints and all loads went into half inch groups consistently,,,Now whats that all about eh!!???.......will have to repeat this another day but it does leave you scratching your head........O

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"Now what's that all about?"  Pure and simple - the wind.

I've shot competitive benchrest with a 22PPC and 6PPC for 25 years, using proper machine front rest, rear bag AND WIND FLAGS!

Even so, I'd be fibbing if I said I'd never shot a half MOA group and so would every other benchrest shooter on the planet!

 

 

 

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What Vince doesn't mention is that if there are 21 wind flags spread around the 100 yard Diggle 'A' range on a BR comp day, it's not that unusual for 7 each to show left wind, right wind, and can't make my sodding mind up which way it is wind! Alternatively, they change aspect so fast, it takes lightning reactions to get a shot off after a glance at them before they reverse.

That's why a 0.1-MOA rifle and shooter like Vince and his kit sometimes gets a half-MOA result!

(I can beat half-MOA anyway, anytime there - Yah Boo! My groups are bigger than your groups!)

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OP I haven't read all the responses here yet but in my 6.5x47L I single load 139 scenars that have half a bullet diameter in the neck hasn't caused any issues for me gun has shot some very small groups.  On the positive it allows you to load more powder for more speed if you want or same powder and less pressure.

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I said I'd come back on this issue, but having researched it as much as I can, there is very little around on this matter of ogive form and why some jump levels work and others don't.

If you take Bryan Litz's 2009 words as copied into my post on March 2nd, AFAIK there hasn't been any progress here - in other words, we didn't really know how or why seating depths affected precision back in 2009, and still don't know today.We just know that it does and that it matters much more or less with some shapes of bullet than others.

Here's what Bryan Litz says in Chapter 16 ('Anatomy of a Bullet') of his original edition of Applied Ballistics for Long Range Shooting:

After explaining the difference between tangent and secant form ogives and how they affect drag, and the importance of nose length as opposed to shape, so far as this (precision and seating depth) aspect goes Litz says:

............ Almost all ogives are circular arcs. The circular arc will be either tangent to the bearing surface or secant to the bearing surface. If the arc joins the bearing surface smoothly, it's called a tangent ogive. If there is a visible abrupt junction between the bearing surface and ogive, then the ogive is secant

He explains the optimum drag-reducing nose profile for a secant ogive is double the radius (in calibres) of a tangent equivalent (ie same nose overall length). So if a tangent design has a nose radius of 7 calibres, the secant type is optimised at around 14 calibres radius. The Rt/R ratio that is a shorthand / guide as to what type a bullet is, is the ratio of actual nose radius to that of a true tangent form, so if a given length nose in a given calibre is tangent at 7-calibres radius profile, a true VLD secant form will be 14-calibres and its Rt/R is 7 over 14 = 0.5. If it has an 11-calibre radius, the Rt/R value  increases to 0.64 and if it is a true tangent form it is 7 divided by 7 = 1.0.

So a combination of a long nose and a high-value nose radius gives the best result in terms of drag in supersonic flight. But the downside is the seating depth / precision issue. So far as just the profile is concerned, Litz says that the secant type can reduce drag by as much as 12% over tangent for two designs with the same nose length. The downside is the abrupt shank to bearing surface junction in a secant ogive bullet, of which Litz says here:

..... Bullets with secant ogives are known to be finicky in relation to seating bullet depth. A popular notion is that the more abrupt nose/body junction for the secant ogive doesn't self-align itself in the riflings as well as a tangent ogive bullet. A common solution is to soft seat the secant ogive bullets into the lands, thereby eliminating bullet jump, and minimizing the potential for misalignment as the bullet starts down the barrel. ..................... He goes on to say it is also more difficult to manufacture consistent bullets with secant ogive profiles and very high quality jackets are needed to avoid wrinkles.

So, there isn't a great deal of science at play here. This is empirically gained knowledge - we know this works or doesn't, but the reasons are mainly supposition. This of course is very much the case too with traditional FMJBT or FMJ tangent designs whose origins are all military and result from huge amounts of on-range testing of various designs and tweaking of designs a long, long time ago. If the research work was ever published at all, it's impossible to find it now. The Swiss (Major Rubin) were in the lead here and the 174gn FMJBT as loaded in the 7.5X55mm  GP11 was an outstanding long-range bullet for 1911 (and still is which is why the Swiss still use it in the MG51 machinegun). The Finns followed in the 20s and 30s with the D-series rebated boat-tail bullets which are also still in use today. The objective of the D-series was bullets that reduced barrel wear compared to a conventional FMJBT, were less affected by it and which retained dynamic stability in ELR shooting of 4,000 metres plus. (Not the same thing as BC whose maximisation  sees  our modern super-long pointy designs, but which become prone to instability at ELRs.)

Just how good the D-series is with regard to shrugging off throat wear (and hence presumably misalignment) can be seen in the reprints at the bottom of this Accurate Forum topic page with embedded copies of a Finnish Army trial with the 7.62X53R which primarily looks at bullet size / match to the barrel but tells us more. For the full-size bullet, precision peaked, (ie average group size was at its smallest) at 14,000 rounds down the barrel, and thereafter steadily rose. You can correlate throat wear to precision through the two graphs. One wonders how a modern VLD would have coped here. (Badly one imagines!) The other thing that I took from this was how good barrel life is with a mixture of a sub 45,000 psi cartridge and chrome-moly steel barrel.

http://forum.accurateshooter.com/threads/243-barrel-life.3934900/page-2

 

In the past we had two designs - tangent (high drag but seating depth tolerant) and VLD (low drag but seating depth intolerant), then we got a third type, the Berger Hybrid that sees a secant profile ogive transition into a tangent type seamless bearing surface to nose junction. Problem solved, best of both worlds. But is it so, and for that matter is it a new design?

As to the latter question, the Hybrid is actually a VLD, but with a 'fix' to try and overcome the VLD's main disadvantage. I'm not at all convinced the 'fix' always works ... or even works at all! The few models I've used 1) aren't at all jump tolerant like a 'traditional' Berger BT or Sierra MK tangent design, and 2) seem to vary (markedly sometimes) as to what works best. For instance in 30-cal, the 155gn Hybrid only shoots well for me in 308 Win with a large (40-50 thou') jump. In the lands or 10-20 thou' jumps give very poor results. The 168gn Hybrid won't shoot with normal jumps for me but needs to seated in the lands like a common or garden non-hybrid VLD. (I'm not alone with the 168 and have this finding confirmed by others.)

As I pointed out in my post of 1st March, Berger's flagship 308 bullet now is NOT a Hybrid, but a long-nose tangent design whose Rt/R ratio is 0.97 and as close to true tangent as you'll get. The nose length is 0.848" out of a bullet OAL of 1.508" and with a short bearing surface in relation to the other dimensions of 0.413". That nose length is the same as those of the 215 and 230gn Hybrids give or take a few thou' and identical to that of the new 225gn Hornady ELD-M. A very long bullet with a very long nose, and I have heard rumours that some wind conditions can throw the 200-20X performance right out at very long ranges, presumably as a result of its approaching or entering transonic speeds at 308 Win MVs and maybe suffering stability issues due to its length. (If the bullet is very / over long in relation to its calibre, it risks becoming dynamically unstable at extreme ranges, and spinning it faster through a tighter rifling twist will reduce that tendency a little but won't cure it as spin-rate affects gyroscopic stability, a different aspect of bullet behaviour.)

Having spent a lot of this miserable winter studying Litz's third edition of his book Ballistic Performance of Rifle Bullets (over 950 bullets now described and tested), I've decided that going past headline form factor and BC figures is well worth doing and looking at bullet shapes and dimensions shows that many of the 'rules' about company A making this kind of bullet and B the other have been torn up and thrown out the window. The manufacturer will trumpet BC all day in its literature and website, but what about bullet and in particular nose profile?

I've praised the 0.284" 160gn TMK a lot here and elsewhere. Lovely bullet, good if not outstanding BC, very easy to tune and it stays 'in tune' over a lot of rounds down the barrel so no 'chasing lands' here. Look at it and it is a reasonably long-nose tangent type whose Rt/R is quoted as 0.84, same as the old 'traditional' form 175gn SMK that I consider an outstanding and much under-appreciated 7mm bullet.

So ... (foolishly) I assumed, all the new TMKs would have similar characteristics. NOT necessarily so.

0.224 69gn TMK ............... 1.00

0.224 77gn TMK ............... 0.96

0.243 95gn TMK ............... 0.81

0.284 160gn TMK ............ 0.84

0.308 155gn TMK ............ 0.55

0.308 168gn TMK ............ 0.54

0.308 175gn TMK ............ 0.55

0.308 195gn TMK ............ 0.46

So, you get nice seating depth tolerant designs in 22, 6mm, and 7mm, but when you get to .30 you find a quartet of out and out VLDs. (Did you wonder why you've maybe struggled to get the new 175 to shoot well when the old SMK of the same weight is such a sweetie?)

 

With Hornady, you assumed usually correctly, you always got a secant quasi-VLD type with the Amax models, how about the new ELD-M designs? (Amax Rt/R in brackets after the ELD value).

0.224 73gn ELD-M ............. 0.67 (-)

0.224 75gn ELD-M ............. 0.48 (0.68)

0.224 80gn ELD-M ............. 0.50 (0.67)

0.243 108gn ELD-M ........... 0.91 (0.74 105gn)

0.264 120gn ELD-M ........... 0.57 (0.56)

0.264 123gn ELD-M ........... 0.86 (0.43)

0.264 130gn ELD-M .......... 0.67 (-)

0.264 140gn ELD-M .......... 0.84 (0.72)

0.264 147gn ELD-M ......... 0.81 (-)

0.284 162gn ELD-M ......... 0.50 (0.55)

0.284 180gn ELD-M ......... 0.60 (-)

0.308 155gn ELD-M ......... 0.55 (0.57)

0.308 168gn ELD-M ......... 0.51 (0.58)

0.308 178gn ELD-M ........ 0.85 (0.56)

0.308 208gn ELD-M ........ 0.81 (0.68)

0.308 225gn ELD-M ........ 0.67 (0.69 - 225gn HPBT Match)

So, a mixed picture for this make. The 22s (75/80) have become very much more VLD type; some have stayed the same; but a significant number should now be much more jump tolerant than their AMax predecessors, in particular the 108gn 6mm, 123gn 6.5 and the 178gn and heavier thirties.

MOREOVER ..... Hornady really has reduced drag / increased BC in its ELD range as the company claims. Despite the doubling of the 123 6.5's Rt/R value, the new model has an improved BC. The new 162gn 7mm has an outstanding BC for its weight - shame it's an out and VLD, but I've bought 200 anyway to try in my 7mm-08 and 284 F-Class rifles. The 140 and 147gn 6.5s have both good Rt/Rs and low drag shapes. (The 140 AMax BC increases from 0.288 G7 to an outstanding 0.322 which is higher than the Berger Hybrid's; the 147gn is rated by Litz at 0.335.)

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Great work there Laurie and a very informative read.  Many thanks indeed for taking the time and trouble to post this.

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