Possible 300 blackout case head separation?

[matm]

Hello everyone! I have been reloading for my AR in 300 blackout for a bit with no problems and relatively low pressure loads. Brass all looked good except a little beat up at times from the action. Recently I got a brand new ruger american ranch rifle in 300 blackout also, never had a bolt action in this round before so I wanted to do some load development. I fired a bunch of rounds with different charges but nothing too hot. The hottest was 17gr h110 with 147gr m80 seated to 2.150 no Crimp. They all fired fine didn't seem hot, no sticky bolt or anything like that. I got home and was looking over the casings and some show a slight bulge just above the base of the case. Primers aren't flat or anything else that would indicate excessive pressure. Most of it is my own formed brass but I saw the same bulge on some of the brand new factory nosler varmageddon. Even some of the lower charge cases showed this (well below max) Ran the paperclip test didn't feel any separation. Do you guys think it's the chamber of the gun or should I scrap the brass and try again? Any help is appreciated ! Thanks

-mat

 

[bigedp51]

Your cases are just expanding outward to meet the chamber walls when fired and this is normal.

What you are seeing is more pronounced when the case is at minimum diameter at the base just above the extractor groove.

Below both of these .303 British cases were fired in the same No.4 Enfield chamber. The HXP case on the right was a smaller diameter at the base and expanded more than the Prvi case on the left.

I would use Lake City or other military 5.56 cases for conversion to .300 Blackout. Military 5.56 cases are made of harder brass than commercial .223 cases and would look more like the Prvi case above when fired.