Wide deviation in speed

[simonl]

Hi,

 

On my first load dev with 243, 58 gr vmax in new lapua brass, I've got a significant variation in MV across *some* of the individual loads.

Some are within 1% variation, yet some have over 5% difference between max & min.

4 cartridges per charge.

 

I believe I'm well clear of the lands - I copied the length of some hornady factory ammunition.

Caldwell optical chrono.

 

Suggestions welcome. Novice reloader but with a technical background. I can upload graphs and std deviation etc if any use.

[Chickenthief]

Neck annealing!

[The Gun Pimp]

Never annealed a neck in my life - for what it's worth!

 

Getting small (no more than 25fps) spreads is simply a matter of choosing the correct powder for the cartridge and getting the load right.

[Laurie]

Simon, talking % ES values means nothing to most of us. Since most cartridges / loads run from the high 2,000s to low 3,000s fps, absolute values are far more meaningful. The norm for long-range match shooters is to get the ES of a five, better still ten-shot string under 20 fps. The US Palma teams persuaded Lapua to produce small primer, small dia. flash-hole 308 Win cases to get existing ES values with 155gn Sierra MKs and H. VarGet frrom the 30s to the teens. On the other hand, I know FTR national league shooters who expect and get single figure ES 5-shot strings in the cartridge.

 

For many if not most shooters, low ES values are frankly irrelevant. They simply don't affect performance at shorter ranges. All that matters here is group size, or to give it the proper name firerarm + ammunition precision. In a series of test combinations, it's often a large ES value combination that produces the smallest groups - this is down to the action and barrel harmonics, a 'sweet spot' that copes with a large diversity of pressures and velocities.

 

So far as target shooters are concerned, ES isn't a real issue at shorter ranges. Many 6PPC BR rifles and loads produce dire figures over the chronograph, but win matches with 0.1 and 0.2 inch groups at 100 yards and equivalent at 200. So far as TR, tactical and even F-Class / FTR are concerned, relatively large ES figures (into the low 30s) don't affect scores at up to 600 yards. At 800, 900, and even more so 1,000 yards, it's a different matter.

 

Now, why do we have these differences between cartridges? There are many factors and unfortunately, they mostly interract so it's very difficult to isolate causes and effects. As The Gun Pimp says, the choice of bullet and powder are the usual suspects, Having just loaded five different 175 to 178gn .308 bullets in .308 Win with the other components and charge weight ranges the same, I got a considerable variation in ES figures and patterns.

 

However, there are LOTS of factors - ignition is a biggie (covering primer choice, even lot number; flash-hole diameter and uniformity, primer pocket depth and shape consistency); case quality and age / neck plus shoulder hardness; bullet consistency, amount of jump; sizing methods and amount of resulting case-neck tension .......... and around 20 other factors.

 

The list goers on and on. The trick is to get a combination that works for you and your purposes and not to lose sleep over technical niceties. Smallarms are full of what Dr. Geoff Kolbe described to me as 'non linear effects' and nasty interractions. Even today, after 30 years of ruling BR matches, people still discuss and dispute exactly what makes the PPCs so much better than any other design. Thne most likely factor is not case shoulder angle or overall case taper and shape rather the ratio of case capacity and charge size to bore and bullet weights employed.

 

Unlike large calibre stuff like 105mm field guns and 120mm tank cannon, you can't stick a load of factors into a computer and get it to do all the hard graft for you in design terms. That's a frustration of our hobby, but also a large part of its appeal if you're an inveterate experimenter. Equally, don't try to apply an over scientific approach to the subject or you'll give yourself a great deal of heartache. Listen to wehat works for others, read some good books and see what people say on forums. In addition to this one, look at the US Accurate Shooter forum and daily bulletin. The empirical approach plus a lot of research usually works well, but not always as every firearm, every barrel is different and what works for one may not for others.

[Gandy]

Neck annealing!

Possible but as it's new lapua brass not the most obvious culprit.

[6mmBR]

Neck annealing!

That's certainly not going to be an issue with new Lapua brass as it's already annealed.

 

ES/SD may well improve after the brass has been fireformed to the chamber.

I've found that with my .300WSM it's performance improves with formed brass.

ES drops from 15-20 fps with new brass to 5-10 fps with once fired.

Also inconsistent neck tension can be a issue with new brass so I wouldn't load straight from the box.

I've found over the years Lapua brass tends to have excessive neck tension from the factory when new.

I run an expander mandril into new brass then re neck size with the appropriate bushing this also helps to improve consistency.

Cheers

Dave

[Jagged 77]

Excellent post Laurie. Personally results on target are the only thing I go by, I don't use or own a chronograph.

 

Also agree with 6mmBR on new Lap brass, neck tension has been stiff to say the least on some of the new lots I've had.

[gbal]

Simon,

Just to add a touch to Laurie's excellent points:

 

For a 243 with 80g @3000fps,the elevation 'error' from a 10 fps deviation is .16 inch more drop at 300y.

It will be similarly negligible for your 58g Vmax @ 3750(approx).

 

(A great combo,by the way-given accuracy,it is as good as/better than most out there,for practical purposes,and just needs buying the bullets-how easy is that !)

 

gbal

[simonl]

Some great comments & learning thanks. Here's my results, in absolute values. As you can see, I'm not chasing perfection - I'm an order of magnitude away from it currently!

Powder = TR140 with CCI primers.

 

Charge Spread (FPS Max-Min across 4 shots)
40.5 42
40.8 126
41.1 151
41.4 144
41.7 95
42.0 24
42.3 51
42.6 62
42.9 42
43.2 14

 

I did mention whilst loading that some of bullets needed more force for seating than others & that the necks were tighter than expected. I had assumed that the Lapua brass was perfect. To what extent might barrel temperature fluctuation affect speed? Significantly?

I now appreicate that there's a swathe of factors which will impact speed, but I'm wondering if there's some big/obvious wins.

Many thanks, S.

[6mmBR]

 

I did mention whilst loading that some of bullets needed more force for seating than others & that the necks were tighter than expected. I had assumed that the Lapua brass was perfect.

Many thanks, S.

I did touch on it in my reply but I think you've probably answered your own question?

Try it again with once fired brass!

[jcampbellsmith]

Hi,

 

On my first load dev with 243, 58 gr vmax in new lapua brass, I've got a significant variation in MV across *some* of the individual loads.

Some are within 1% variation, yet some have over 5% difference between max & min.......

 

Simon.

 

Like many things, there's a learning curve to reloading and despite the availability of excellent advice (above and elsewhere), hard work on your part is required to move up the learning curve. One way to accelerate your travel up the learning curve is to compete, but beware, whatever you do, it won't be cheap.

 

Remember to take copious notes on what you do. I still find my own notes are sometimes inadequate when I look back through them and try to work out exactly what I did on the brass prep for a specific load.

 

Have fun.

 

John

[Laurie]

I did mention whilst loading that some of bullets needed more force for seating than others & that the necks were tighter than expected. I had assumed that the Lapua brass was perfect.

 

 

 

Lapua brass is usually very good out of the box, and is certainly more than good enough for loads to be used in 'ordinary' sporting shooting. It does come with necks set at a too small diameter however, so running new brass over an expander is always a good idea. Even at that, it usually produces more neck tension than I run at with my match loads.

 

Your pattern of charge weights and ES values suggests the lower loads didn't suit the bullet / MV in that barrel. That's a separate issue from running within safe pressures, so one shouldn't necessarily keep ramping loads up irrespective. However, whilst I don't have TR140 in my version of QuickLOAD (Reload Swiss RS50), I've found it to have very similar characteristics to Viht N140 in other cartridges, and QL says your starting load was way down around 45,000 psi and 43.2gn was just getting you into the proper working pressure zone at ~54,000 psi.

 

However, that almost certainly overstates the case as you are loading a very light and hence short bullet for the cartridge. Assuming you are loading for a factory sporting rifle rather than a custom built example chambered with an ultra short throat to suit this bullet design, the 58 Hornady will have made a very substantial jump before it reached the rifling. This lowers pressures and MVs and in my experience of playing with 55 and 58gn class varmint bullets in a standard chamber 243 Win and a long-throat 6mm BR Norma chamber some years ago, MVs were nearly always well down on what was expected, precision could be and usually was excellent, but the combination tended to be finicky with relatively narrow 'accuracy nodes' and lower charges rarely if ever produced good results. Your very large ES values for substandard pressure combinations were in line with what I expected.

 

Remember, if you are loading for a factory rifle, the manufacturer has to be sure it'll physically chamber and not run at unsafe pressures with whatever factory ammo is available with the heaviest bluntest bullet and hence the longest CBTO values without the said round's ogive being hard jammed into the rifling and sending pressures through the roof. Moreover, if it's an American made rifle or a European model that will be sold in the USA, manufacturers have become very scared of product liability lawsuits, so have tended to increase freebores in recent years, to the point where some now run at levels straight out of the box that might have applied to a shot out barrel at one time. While 243s handle 55gn class bullets very well in most cases, they're built and designed to handle much longer 85-100gn projectiles as the norm. As a result, they can be very picky about light bullet diameters, bearing surface lengths, and in particular ogive shapes, and simply never work well with some models.

[Elwood]

Caldwell optical chronograph, are they known for great accuracy?

[simonl]

Caldwell optical chronograph, are they known for great accuracy?

At $80, I doubt it! It showed consistency with factory ammo though.

 

Laurie, thank you for those comprehensive posts, very informative.

 

I'm about to follow up with a repeat exercise with 75gr, which I loaded at the same time as the 58gr.