.223 primer blowback/gas leakage.

[markm87]

You didn't try Federal brass with different primers so it's not a complete experiment.

I actually did load 20 rounds of FC with other primers on a prior shoot. The problem seemed to go away, but it was another random load. So not exactly scientific.

 

[markm87]

Which Tula primer are you shooting? Numerous posts on this forum state that experience shows the KVB223 are softer than the KVB556.

Maybe the issue is a combination of softer FC brass with this primer?

KVB-223M. I do think its the combination... AND the sub par FC brass.

 

[markm87]

Punch out the primers on the rounds with soot and examine them and the primer pockets with a critical eye. Seat a new primer in the same exact cases and note the required effort.
The answer is here. You've just overlooked it.

I may do that. At this point I've been tossing the stuff into the scrap bucket after one firing. But it wouldn't hurt to try a handful like you said.