Problem 6ARC Brass from 7.62x39

[WestTN2288]

Ok so I have a batch of a couple of different name brands of 7.62 brass than I have converted to 6arc. However, when I get anywhere near (~1-2grs) a load that I normally use in 6arc brass I start blowing primers in the 7.62. This happens across multiple name brands of 7.62.

I don’t have a chronograph, so I can’t compare velocities, but my question: is the brass causing that much more pressure that I need to be 2gr low? Or is the primer pocket on 7.62 brass that bad?

I like the 7.62 brass because when it works, it shoots good, and there are certain times I want to shoot a lot and not feel like I have to go back looking for brass.

 

[ridgeline]

Weigh case capacity via mass, as it's often eye opening. Increased mass, means less internal volume.

 

[WestTN2288]

Well I have, and some of it is up to 8% higher, while some of it is almost identical. I know weight does not directly equal capacity, but I have checked that. Guess I need to fill cases with water.

Also, the 7.62 brass is large primer vs small primer 6arc. Would that contribute?

 

[WeiserBucks]

Did you turn necks? I would think that necking down from 7.62 to 6mm would result in very thick necks that could spike pressure.

 

[WestTN2288]

I did not, but from checking with my calipers (not sure the right way to check neck thickness), they are within .002