.308 accuracy info

R

Robbert

Guest
I have just bought a new Barrel for my Mauser M03.
caliber .308 varmint match barrel.
Fireforming new brass gives nice groups (sub moa)
However using these brass again gives terrible groups.

what do I probably do wrong?

I used Wilson neck and bullet seat dies.
Barnes bullets 150 gn tsx
Norma brass
Federal 210 M primers
43 gn of N135

thanks!
 
Wag

Trim a few of the fired brass to a length of 2.000 and try them.

Normal trim length would be 2.015. An extra .015 isn't much.

One other thought: I do not get great results using bushing sizing dies with non-turned necks. For non-turned necks the best I have found is the Lee Collet die.
 
Last edited:
To Robbert

If you get good groups one day and not so good the next, be very careful about the overall length of your loaded rounds, because if both batches are not identical accuracy can suffer. Make sure your new barrel is very clean and free of copper fouling and be sure to use wind flags. I don't have much faith in gadgets that make your rounds more concentric as the gains they give are almost impossible to measure.:D
 
I have seen this caused by neck tension not being the same as new brass. Something worth checking out, Good Luck!
 
Thanks for your tips,

neck tension is certainly different. so I will look into that.
Also neckturning will be worth doing I'm sure.
 
Do a little test . full length size the brass again and see if the accuracy returns. I have seen this happen with a crooked chamber . What can happen is that the new loose fitting brass lines up reasonable in the chamber but after fire forming it is bent and tighter fitting causing the projectile to be more out of center than before and have in bore yaw.
The fact that the barrel is new is suspicious.
If you get consistently better groups with new brass and full length resized brass than fire formed cases then I would have someone do some eccentricity tests on your new and fired brass .
If the fired brass is worse than the unfired then you have a crooked chamber.
 
I've frequently found that when a rifle gives good accuracy with new cases, but not as good to poor accuracy with fired neck sized cases that full length sizing helps considerably. With a bump gauge adjust your full length sizing die to push the shoulders back 0.002" (0.05 mm), and try those.

There are a lot of things that can cause poor accuracy, but with a SAAMI or factory sized chamber FL sized cases frequently work best. Lots of possible reasons, but it works usually.
 
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