I recently picked up a new Pioneer Arms PPS-43C in 7.62x25 Tok.  I made a trip to the range last week and everything functioned well and was very pleased with how it shot.  However when I checked some of the ejected brass I noticed that the primers had bulged a bit and pushed into the firing pin hole.  I did not find and pierced or loose primers.  Ammo is new PPU FMJ.

Am I being overly concerned or do I need to consider sending it in to get checked out?

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Open bolt subgun to closed bolt semi conversions always concerned me.  The subgun counted on the forward inertia of the moving bolt to contain pressure.  The bolt would need to be considerably heavier to have the same inertia starting in the forward position.  The various conversions are usually operating right at the edge of safety.

"Cratering" of the primer is not uncommon in simple blowback guns shooting hotter rounds (hotter than .380acp). If that is the ONLY indication of an excessive pressure issue, I wouldn't worry too much about it (but that's just my opinion). If you are also experiencing bulged cases, split cases, torn rims, stretched brass, violent extraction/ejection, etc. (you already said no pierced or lost primers) then you should be more concerned.

FWIW- New production PPU 7.62Tok is pretty mild compared to military surplus.

I don't know how the Pioneer Arms offering was designed but I am guessing there is a striker assembly with a striker spring that is adding inertial resistance to the closed bolt. These are new production guns IIRC so maybe they increased bolt mass as well, IDK. Does it all balance out?

You are correct though, the open bolt to closed bolt re-engineering changes the dynamics of the action completely.

I did not find any split or cracked cases and I am not seeing stretching of cases when compared to unfired rounds. The bolt on the semi should be a little lighter due to the lack of a lip on the tail, a machined channel in the top and the firing pin channel.

Thanks for the feedback.

Firing pin hole is oversized

Maybe, maybe not.  Could be as simple as the radius of the firing pin is allowing the primer to flow into the space available when the firing pin is pushed back into the bolt by pressure.