OCW testing and brass variable?

[thefitter]

Working up OCW for a .308

Shot 5 different loads Saturday. Picked best 2 from Saturday and loaded that for Sunday.

Sunday was crap! Big difference in groups.

Everything was loaded the same except for the brass was a different batch. The brass for Saturday was Federal factory GMM and the brass for Sunday was Federal Military grade.

The brass was processed, sized and trimmed EXACTLY the same and has been reloaded the same number of times- 2 or 3.

What am I missing? Thanks

 

[Victor N TN]

Re: OCW testing and brass variable?

There is a difference between military spec brass and commercial brass. The internal dimensions will differ enough to throw an accurate load off. Next time compare apples to apples.

 

[zack-s]

Re: OCW testing and brass variable?

Yep different case capacities causing difference in pressure

 

[kombayotch]

Re: OCW testing and brass variable?

Here's an example with two different brands of cases:
http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=1087404#Post1087602

 

[vman]

Re: OCW testing and brass variable?

Ya big difference.

To give you an example, even if you buy brand new virgin cases and shoot those, results will differ on the second and sometimes third firing of this same brass.

Virgin brass is not formed to your chamber, but on the second firing is closer to being fully formed.

So if the same batch of the same type of brass can give different results based on the fact that its been formed or not, imagine what difference you will get using different types of brass.

Each manufacturer makes their brass to different specs. They all look the same, but internal volumes will differ.

 

[thefitter]

Re: OCW testing and brass variable?

Well I'll give it another shot this weekend with the same brass. So once this batch is all shot up I have to start all over again? Great...