Alright so I go and shoot two 5 shot groups today in .308 and was extremely disappointed with the muzzle velocity spread.

1st string
2496
2482
2530
2482
2496

2nd string
2520
2510
2492
2520
2501


I'm using once fired Lapua Brass, ultrasonic cleaned, BR-2 primers, Lee Collet neck size die (std. mandrel size) and then redding body die, and a forster seating die seating 178gr a-max bullets to 2.805" OAL on average.  I do a very very light crimp with a lee factory crimp die.  My base to ogive measurements on all rounds was +/- .001".  I measured my charges to within 0.02 grains and went back and remeasured them 3 times.

Could I possibly be getting this much spread due to poor neck tension?  I don't know what else it could be...

What was the SD and ES?
They don't look too eratic.

AVG = 2502.9
STDEV= 16.5
6 Sigma ES = 99.2

Not the best, but far from the worst. What kind of rig are you using?

Are your primer pockets uniform depth? Firing pin and ignition uniform?

Try one batch without the neck crimp just to be sure.

I think what we have here is an expectations problem, not an ammo problem.

How was the accuracy?  What distance did you fire at?

What powder type and charge weight?


average = 2503.7 fps
ES = 48.0 fps
SD = 17.3 fps

It is a semiautomatic.  charge weight was 43.0gr.  I did prep the brass using a primer pocket uniformer and deburred the flash hole.

I chose the collet neck die because it seemed a bit overboard to turn the necks and then using a bushing die.  

I was shooting at 100 yards, but want to shoot and longer ranges.  I did group well at 100 yards.  My concern was that the variation wasn't adequate for longer distances. +/- 25fps amounts to +/- 10" at 1000 yards.

What kind of chronograph and what weather conditions?  Honestly, the numbers don't look terrible.

When you SS tumble brass, you may clean it too much, leading to brass-on-copper galling when seating and firing.  Use some graphite lube when seating bullets for more uniform release and hopefully more uniform velocity.

Trim all cases to the same length.

Lapua brass doesn't need the flash hole deburred, as there is no burr.

Finally, .2gr more or less powder might give more consistent velocity.

I'm not doing any SS tumbling.

Chronograph is:
http://www.midwayusa.com/product/852429/competition-electronics-prochrono-digital-chronograph

It was partly cloudy out, it always registered a shot.

Yes you'll want better numbers for long range. Obviously the lower the better especially with a .308


You don't need to neck turn with Lapua brass. IME it is very consistant. I am using a Forster Bump Neck Bushing die for .308 and .223. I measured several neck walls of the Lapua and found them to be as good as expect one could get neck turning.

DO NOT CRIMP! for accurate ammo that you want to send long distances, I don't think you will get your numbers down crimping. If you need more neck tension for feeding from a mag you would be better off sanding down your mandrel or go bushing die  for the tension rather than crimping.

Stop crimping first, if that don't help try messing with charge weight. For long range you are better off with a load that has good SD and ES numbers and not so good groups than good groups but shitty SD and ESs.( with slower loads and low hit probabilities)


Off on a tangent I go to explain my above statement.......

For example:(from my Applied Ballistics WEZ analysis program)

17fps SD with .75 moa Rifle Precision equals 69% hit probability(in zero wind on an IPSC silhouette @1000)

10fps SD with 1.25 moa Rifle Precision equals 84% hit probability(in zero wind on an IPSC silhouette @1000)

If you can keep the group size and lower SD to 10fps SD with .75 moa Rifle Precision equals 89% hit probability. Don't help much.

.75moa vs. 1.25moa is a difference of 5" in groups size at 1000yds. That is a 2.5" to the left, right, up and down, nearly meaningless considering drift is 11.5"/mph of wind at that range. 10mph of full wind is over 110" of drift, that 2.5" one direction or another is insignificant.

If I can get 3/4 to 1moa at 100yds with 10fps or less SDs and 30fps or less ESs(over 10-15shots) I call it good. I am shooting .308s and .223s out of short barrels to longer distances than the rounds are ideal for, where wind drift is the largest factor. Wasting time trying for tiny groups is pointless.

I didn't see what powder you were using. I rarely get great ES and SD when loading IMR-4064 in .308, however accuracy is usually very good.

For short range, anything inside of 300 yards, it won't make any difference whatsoever if you have high numbers on your ES and SD. Even double or triple your current numbers won't appear on target at short range.

Some of my most accurate 100 yard loads have had unacceptable ES and SD numbers which I would have been ignorantly blissful of had I not had a chronograph.

BTW - I think your posted numbers are pretty good and perfectly acceptable for 600 yard tournaments provided it shoots tight groups.

I never crimp match grade bullets and since I only use match grade bullets that means I never crimp period.

Varget

 
For long range I agree with this.  Not sure what I'd try next but, my lowest SD and ES numbers tend to be with the loads that have the case the most filled up.  I'm not a big fan of the Lee Neck die for precision stuff either.  I also don't trust a lot of chronos on the market to give numbers that are 100%

I'm basically near 100% fill at 43gr.  If I vibrate the charged cases before seating it leaves a little bit of extra room.

I'd bet the numbers get better without a crimp.