Need help. 6 CM. Bad Carbon blow back and weird case results.

[hunting_fishing_addict]

This is a GAP PPR in 6CM. Bartllien 26" 1:7.2

First firing for this brass.

Using: 40g H4350, 205M, Peterson SRP, .070" jump on 105 Berger Hybrid

First off. The rifle actually shot well. It was shooting .2 inch groups at 100y. And the SD was 6.

But I was getting some weird results with the cases.

This is my virgin brass:

1.478

This is a fired case with a carbon build up on the shoulder and a little on the case body:

1.480 / .002 expanded over virgin brass

This is a fired case with carbon build up all over the case, even heavy on the extractor ring:

1.471 / .007 under the virgin case

This is a fired case that came out clean:

1.486 / .008 over the virgin case

Why is this happening? Do I need to seat the bullets closer to the lands to increase pressure? Do I need to increase powder charge to increase pressure? Changing primers is out of the question lol.

Thanks,
Seve

 

[Doom]

Based on what I see I think you may have two problems. First is the shoulder may be set back too much for your chamber. The second is, as you have noted, appears to be low pressure. I would look at increasing the powder charge.

 

[Jumper]

Deprime the cases and remove any burrs from the case head and measure again. To remove burrs, I use sand paper on a flat surface. A couple passes removes most of the bumps.

 

[hunting_fishing_addict]

I’m gonna run up the powder charge to 41g and seat the bullets to .020”, taking it out in the morning, I’ll let you know. Thanks.

 

[hunting_fishing_addict]

I still have this issue. It goes away and the cases expand when I increase pressure by either jamming the bullets or seating the bullets deep in the case/longer jump. It doesn’t matter if it’s a magnum primer or match primer, the case will not expand if the bullet is between .020”-.70”. My next step is to try annealing the brass.