TMKs - Seating distance off the lands

[VarmLR]

VarmLR

 

what you are actually doing when you change seating depth/ powder load is altering the point in the oscillation of the muzzle at which the bullet leaves the crown, so there are multiple combinations of the 2 variables that will work, just as there are multiple velocities/loads that mean the bullet exits at a minimum oscillation node( in a 22" barrel these seem to equate to 2650fps ish and 3000ish from my experience, although there may be ones in between). What the OCW method seeks to do is give you a guide as to when the muzzle is at it's least amplitude of movement. That's why small changes in temp/load etc around that node still shoot to essentially to the same poi.

Forgive me if I'm teaching my granny to suck eggs, but so many people bang on about "these bullets like to jump this and those ones that etc etc " when all they are actually doing is tuning the exit point of the bullet from the barrel to a zero amplitude node. Once you get your head round this doing load development becomes more of science and less of a black art!

 

Hope that helps

 

Mike

 

PS In those weather conditions it would be risky to draw any firm conclusions, but sounds like you're heading the right way

 

 

Thanks Gun Nut. Absolutely it's a science, and one I'm still keenly learning. I may have been shooting more years than I care to remember, but I've really only got to grips with ballistics since I started down my re-loading journey, so what is obvious to many (most?) UKVarminters is still quite fresh to me but hopefully I'm on the right path.

 

Yes, all I'm trying to establish is to use the OCW approach to find the node of least amplitude, at least with these bullets. I know that it isn't close to the lands or just back from them as 10 thou jump doesn't seem to produce remotely close enough groups, but moving back a bit the group sizes almost half, which points me towards a longer jump needed in my rifle for this bullet. I appreciate that other rifles may shoot differently due to differences in barrel harmonics.

 

A question though....whilst I'm finding that there seem to be two loads that work for most of the bullets that I've tried (echoing what you've said about corresponding velocities) surely the MV that occurs at will depend upon barrel stiffness and length to some extent? I haven't yet chrono'd the loads that seem to work best with TMKs but it might be interesting to do that. Also, I'm assuming that a change in mass of the bullet used might alter this point due to inertial differences altering the amplitude of oscillation and hence the time for the settling of that oscillation to minimum, or will that node remain the same for all types of bullet? (ie find that node and load all bullets to that jump?)

 

Why not try and jam them in the lands. Am shooting 77gr TMKs in a 1:8 twist 24" barrel at 5tho jam. It may work for you!

 

Thanks Sean but I'm finding that most bullets I've so far tried seem to shoot better loaded a little further back than that, plus until I get more experience in reloading and ballstics, i dont want to load up against the lands where slight variations in powder load might increase pressures a lot. Until I have more experience, I would like to minimise the value of any variations and err on the side of caution, safety wise.

[onehole]

Varm I,m gonna make you feel better,,,,,was out with a mate yesterday to check zero before some foxing and in similar wind speeds but shooting almost with the wind the 200 yard groups ranged between 1.1/4 and 2.1/2 inches.The 69 TMK,s seemed to fair better than higher speed 52 Amax,s. Both rifles that were used,,,my Howa mini in 223 and a his Remmy Tactical in 223 will both print 100 yard 1/2 inchers on a calm day with ease. ,,,,,O

[VarmLR]

Varm I,m gonna make you feel better,,,,,was out with a mate yesterday to check zero before some foxing and in similar wind speeds but shooting almost with the wind the 200 yard groups ranged between 1.1/4 and 2.1/2 inches.The 69 TMK,s seemed to fair better than higher speed 52 Amax,s. Both rifles that were used,,,my Howa mini in 223 and a his Remmy Tactical in 223 will both print 100 yard 1/2 inchers on a calm day with ease. ,,,,,O

 

 

 

Yes, I think that the gusting nature of the wind would easily account for the spread that I was seeing. I will be repeating the tests in (hopefully!) calmer conditions this week so we'll see if the pattern pointing towards about a 50 thou jump with these in my rifle translates to tighter groups in calmer weather. 2.5 inches in windy conditions at that range is better than I'd expect and 1 1/4 seems very respectable. Drift for a 69g bullet at 200 yards for say 10mph full I'd expect to be close on 4 inches.

[onehole]

 

 

 

Yes, I think that the gusting nature of the wind would easily account for the spread that I was seeing. I will be repeating the tests in (hopefully!) calmer conditions this week so we'll see if the pattern pointing towards about a 50 thou jump with these in my rifle translates to tighter groups in calmer weather. 2.5 inches in windy conditions at that range is better than I'd expect and 1 1/4 seems very respectable. Drift for a 69g bullet at 200 yards for say 10mph full I'd expect to be close on 4 inches.

 

 

Our groups were drifting a lot less than 4 inches as we were with the wind but definite left to right influence with shots being dispersed horizonally and the main reason for spread.Pretty sure you will see a better result with yours on the right day,,,,,O

[gbal]

Yep: Make/weight/BC/MV 200 drift 10mph;all SAAMI:

 

Rem AcTip 50 .242@3410 4.5

Win BSilTip 50 .239 @3410 4.6

BH 50 Vmax .242@3300 4.7

Umax 50 NosBT .238@3100 5.2

 

Lap 69 Scenar .322 @3030 3.8

CorBon 69 .338@ 3.7

Rem69 HPBT .336@ 3000 3.7

Fed 69 SMK .300 @ 2950 4.3

Nos 69 CC .305 @2900 4.4

BHill 69 SMK .338@ 2850 4.0

 

 

CorBon 77 HPBT .350@2800 3.9

Rem 77MK HPBT .362@2788 3.8

Fed 77 Match .372 @ 2720 3.8

BH 77SMK .390@2750 3.6

Nos 77 CC .340 @2600 4.5

 

 

There are (small) effects of Weight/ BC and MV around the 4 inch 69g mean.

 

gbal

[gun nut]

Varmlr

Away on holiday at the minute. When I get home I'll send you a page of data for barrel length harmonic times. Someone with more brains and equipment than me has worked out the barrel transit time to hit the zero nodes for different barrel lengths. There are five node times for each barrel length. Apparently the weight or stiffness of the barrel only affects amplitude not node times.

Mike

[VarmLR]

Thanks Mike. This is for a 26 inch barrel. Interesting to note that that stiffness may not have an effect on transit times.

[VarmLR]

Ok

 

Out today and tried a few different loads. Settled on 24.7grains N140 as the accuracy load for my rifle.

 

I found two nodes for seating the bullets, and straying even slight outside of them resulted in rapid opening up of groups. In my rifle, the nodes are clearly at 2o thou off and 100 thou off lands as follows:

 

 

 

at 20 thou off,, a respectable 0.39 inches. I think that with a little more care in shooting the group, I could tighten these up a bit further, so the .3moa objective isn't too far away (want a load for longer range gong shooting).

 

The surprise was at 100 thou off the lands:

 

 

 

So there I appear to have my nodes for fine tuning. If I wanted to be more anal, I could venture 5 thou either side of these points, then up or down by 0.1 grains, but pretty happy that these look like being the approximate accuracy nodes now.

 

Thanks to everyone commenting for your input and advice.

[Big Al]

I would run the test a couple of more times, if the results are the same then you have a decent starting point for fine tuning

[onehole]

Gonna rock the boat a bit,,,,,,unless you have to load a million miles off because of mag length why are you doing this? read the findings in the secrets of the Houston Warehouse,,,one has to take notice? surely? loading into lands is not a practical place to be but just off is?I have never had a rifle that cant be tuned in at barely off or 5 or say max at 10 thou,,,why would you want to be anywhere else. If you cant get a group up close its got to be something else going on and there,s a whole heap of things other than seating depth that will mess you up.Pretty much convinced seating depth should be close to lands as possible and given "everything else" is spot on then the correct powder and load will do the rest.

[VarmLR]

I'm not one for loading jammed, but 10 to 20 thou off is usually where I end up for most loads. It's just this time, using these TMKs, for whatever reason (and I don't claim to understand why...yet), I have has some issues fnding an accuracy load but as of yesterday there are two accuracy points evident to me. As Al says, the tests need re-running several times at those two points to see if the results are consistent. If they are, I'll probably stick with 20 thou off. I can't say "because that's what works for me using these rounds in my rifle" yet, because the tests need to be repeatable and using more than 3 shot groups. That was a starting point to highlight significant differences. Still learning and a way to go yet.

[Murph]

You will hit an accuracy node from 25.4 to 26.4 with n140. This is using sierras own load data for the old smks. I have used both and their accuracy nodes are identical with both bullets. These bullets really come to life running at 3000-3100 fps. This is a safe load in my rifle but may not be in yours. Good luck with your load development.

[Murph]

Gonna rock the boat a bit,,,,,,unless you have to load a million miles off because of mag length why are you doing this? read the findings in the secrets of the Houston Warehouse,,,one has to take notice? surely? loading into lands is not a practical place to be but just off is?I have never had a rifle that cant be tuned in at barely off or 5 or say max at 10 thou,,,why would you want to be anywhere else. If you cant get a group up close its got to be something else going on and there,s a whole heap of things other than seating depth that will mess you up.Pretty much convinced seating depth should be close to lands as possible and given "everything else" is spot on then the correct powder and load will do the rest.

My loads are over 200 thou off the lands. I think that the correct amount of powder plays a much bigger part in group size. I tried these bullets from the lands back to almost factory length. I found at factory length that there was slight compression of powder which decreased velocity out of my rifle in the field. Also putting another .2 grains of powder into the case made the bullets drop an extra 6 inches at 600 yards. This was using 25.7 grains of h335. As soon as I tried 25.9 it lost velocity. This told me that there must be unburnt or in efficiently burnt powder threw into the equation. I've shot over 1000 of these bullets and they are very tolerant of distance to the lands.

[VarmLR]

I'm finding that loads are starting to be compressed by 24.7 grains. Haven't chrony'd them yet but will do. COAL was 2.26inches for the TMKs for a 100 thou off the lands seating depth.

 

Don't forget that Sierra's data is for the SMK which is shorter by 82 thou. Loading the TMK to the same OAL as the SMK will therefore compress the load, so the safe load I'm guessing will not be as hot as their findings for the shorter bullet. Only very slight pressure signs as primers shoulders just starting to lose definition with a little flow by 24.7 but no ejector marks nor cratering. Will go up very slightly, 0.2g and try a few at 100 thou seating at that. I'm pretty sure that loading their suggested top two loads for the SMK would overly compress the charge.

[Murph]

My oal for these was 2.360. This allowed my bullets to seat with very little compression of powder. This was using 25.7 grains of h335 which was just below the neck in the case. You will definitely hit a sweet spot over 3000 fps- close to 3100 fps.

[VarmLR]

I'm dropping the load back to the next harmonic (advice from QL data) to 23.6g N140 and trying again going back off the lands. Whilst 24.7 was pretty good, it was also pretty hot....I hadn't made any allowances for shooting later in the yet once we've climbed out of this coolish weather. Another 10 degrees C could see the 24.7 give rise to higher pressures. Want to preserve the throat too, so will persevere. Had a few reasonable groups as 23.6 last time out. It wont make a huge difference to velocity so just need to fine tune seating depth to get time of travel of bullet to coincide with barrel close to on axis node. At the minute, there's a few seating depths standing out as close to that node (between 10 and 20 thou and 100 thou). We'll see what some improved discipline behind the trigger can do at that.

[weejohn]

I have followed this thread since I was planning on using the 7mm TMK in my 284.

I worked up a load last Friday and settled on a jump of 45 thou.

It got me wondering if the jump required is not so much about the bullet but actually what the barrel likes ?? Only reason I wondered that is because the 162 amax also shoots at its best around the same distance from the lands.

[VarmLR]

It is all about the barrel as I'm finding out. This business of what "a bullet likes" I think is nonsense as it disregards barrel harmonics. If you pick a load irrespective of jump (I often start at 10 thou) which gives the smallest 3 shot group consistently, then work back on seating depth, you'll find very obvious openings or closings of groups which in theory correspond to a node where the barrel is close to axial (min node) at the muzzle. It corresponds to the time from the round is ignited to it exiting the barrel. I think it's more commonly referred to as "barrel time". Both the powder charge and seating depth seem to affect this time, as both have an effect on distance the bullet travels or velocity which in turn affects the barrel time.

 

Change the load or the seating depth and this seems to vary, in the case of the 308 by a large margin (I had 6 inch differences in POI by testing loads half a grain apart at the same seating depth, thrown around the clock face). The 223 doesn't seem to show POI differences quite as dramatic but the principle is the same. It is frustrating spending so much time and effort on this part of development, not least due to cost in time and money but there's no shortcuts to be had other than using a systematic and disciplined approach. I have been guilty of being blinkered over the winter/spring and not considering effects of temperature rise on pressure later in the year on loads so have had to reduce my loads to compensate, which has meant starting over!

 

Something I'm in discussion with another UKV member with is shooting discipline. One thing that I've lost practice on having hunted for so many years and only relatively recently (last year or two) gotten back into target, is technique. I think that many new to the sport, and to reloading perhaps(?) may not pay enough attention to is in removing the variable of the shooter from development or grouping practice. I spend a long time with each rifle not shooting at all, just training myself on the best shooting position, breathing, consistency of trigger pull etc. Even slight changes to cheek position or body position will alter results, so until I'm confident that I can be consistent myself,which means realxed and comfortable too, the rest is a waste of time and I know I'll not get anywhere.

 

Sometimes I think that there's no end to this journey, and perhaps that's true. I'm certainly learning something new with every outing and every attempt at load development. What is fantastic though, is the help that those with more experience are only too willing to give and the time that they give up doing so. It makes the shooting community what it is. Some really great people as members of this forum particularly.

[gbal]

Weejohn,in a sense it's always 'what the barrel likes' for precision.

 

If the seating depth relates to travel time up the barrel,won't the 160 TMK be pretty close to the 162 Amax-same powder charge of course-beingvery close in weight,so acceleration should be close (we are dealing with very fine differences-and in theory-in bullet/barrel travel time,though there may be very small effects of surface friction/bearing surface lengths.....probably travel time is directly proportional to bullet mass,and distance. Basically,very similar bullet/weight will arrive at muzzle at the same time. Considerable change in bullet weight 'should' change that,and therefore optimal seating depth...?

 

Not sure there is much definitive research on this; it will be difficult indeed ,even in a lab,to get precise measures-one clue is barrel tuners,which effectively change barrel vibration,to 'match' bullet/load (most are 22rf of course,where bullet seating isn't readily variable-though the BOSS system was cf....whatever happened to that idea?)

 

One 'scary guesstimate' on this approach,is that the useable life of a barrel is around 10-20 seconds !!

 

(number of rounds x travel time for each) :-)

 

gbal

[VarmLR]

That statistic is quite scary George, and I feel that I may well approach 10 seconds barrel time before I finish the load development for these pesky TMKs!

 

Bearing surface length of a bullet may well affect swaging pressure differences, indeed not all barrels are the same in this respect. Some perhaps might be more sensitive than others, perhaps affecting longer bearing bullets of the same weight as their shorter bearing counterparts? It can get all too easy to start over-thinking development though instead of getting out there and trying to use a consistent method to try a sensible number of loads and jumps.

 

I'm guessing that investment in QL can and does shortcut the load dev times and save some £££'s and barrel life so I may well invest in it at some point.

[activeviii]

Less than 10 seconds. A hunting barrle running slower hunting loads is around 10 seconds before is starts opening up. A target rifle running fast but not over heating is more in the 5 seconds and the hot shots can be down to as little as 2 seconds.

 

A report done a little while ago by Mr Macmillan was also along these lines

[VarmLR]

Is there a link to the report available anywhere Phil? Sounds interesting. After having to fork out for a new barrel, I'm minded to run my loads on the low side as I use the 223 a lot, so will concentrate on accuracy loads that aren't too hot.. Shooting mostly inside 600 yds, nothing more is needed. Well impressed with the groups you're managing Phil with the T3 by the way.

[activeviii]

Is there a link to the report available anywhere Phil? Sounds interesting. After having to fork out for a new barrel, I'm minded to run my loads on the low side as I use the 223 a lot, so will concentrate on accuracy loads that aren't too hot.. Shooting mostly inside 600 yds, nothing more is needed. Well impressed with the groups you're managing Phil with the T3 by the way.

My groupings are all luck.

 

I will try and find the arrival and mail it to you. It was a while ago I read it.

[gbal]

Thanks,activeviii- my apologies,I was being frivolous and guestimated around 10secs,then allowed some leeway.....as below.

 

Here is the 'got a few more minutes than I had before tennis yesterday' version,and comments:

 

First approximation is the easy one:if MV is 3000fps and barrel is 20 inches,then bullet barrel time is 1/1500 secs,and if you get 3000 accurate rounds,that's 2 seconds of barrel life !! BUT:

 

The bullet starts at zero and accelerates down barrel,so average speed is not 3000fps (so not 1/1500sec)

 

Second approximation-assume constant acceleration from zero-gives .0011 sec in barrel;but it isn't really constant so takes longer....let's take ,002 sec as a more realistic time(that obviously depends on velocity and barrel length,and probably errs on the side of 'longer barrel time/life',for reduced angst.)

 

SO: 3000 accurate shots at .002 sec each gives barrel life of ... 6secs.

 

AH,BUT: what does 'accurate barrel life mean"? Good question,some go more than 3000,( a 308 eg might give 5000 shots ie a whopping 10 seconds life) some less-a lot less , of course and that still leaves open the criterion for 'accurate'...

....many change their barrels when top precision starts to go-and that is probably near essential for group/score small target competition shooters (.1 moa matters),BUT:

 

many rifles are quite adequate for many rounds (thousands even) when their top precision has gone. Their barrellife is clearly increased accordingly-let's see how this plays out for 'club'/ fun level shooting:

here's the 308win,175 SMK@2600 (assume excellent wind reading to +/- 1mph,ditto range +/- yard,with various size targets at various distances,and three different moa capable rifles -WEZ hits %s are:

 

moa .5 rifle 10" gong 600y 99% 1000y 40% IPSC(cf fig 11) 600y 100% 1000y 86%

 

moa 1 rifle 10" gong 600y 93% 1000y 35% IPSC(cf fig11) 600y 100% 1000y 82%

 

moa 1.5 rifle 10" gong 600y 84% 1000y 25% IPSC(cf fig 11) 600y 100% 1000y 77%

 

now,purely as comment,many casual/club shooters will not shoot beyond 600y,so even at quite a small ten inch gong,will not be aware of a performance difference in any of these three rifles-they are all capable of consistent hits-differnces only emerge over large nubers of shots-like a hundred,and still might well be put down to wind/shooter/etc-remember the hit % data assume no such errors-just moa related,in excellent shooting environment.

 

The same is true for the 'fig 11' type reactive targets....a hit means down,and even at 1000yards the hit %s are close....how many such shooters would really notice (given the other 'reasons' on the excuse list',as seen by the shooter),and even then ,how many such shooters are going to rebarrel for a 5% hit improvement?

 

Sooooo....unless you shoot golf balls 400 yards away,and never miss,you may not notice your precious few seconds barrel life is now forfeit; when you do,you could buy tennis balls, (though my experience last night is by age 70+ they are getting hard to hit well even at short range,though they were dynamic,and incoming-12 guage territory).

 

OR, you could load a tad lighter, extending barrel life by as much as .001045 secs for every shot. :-)

 

gbal

[VarmLR]

double post