[totaldla]

I went back and played with locking and unlocking the slide while shinning a light down the gap. The firing pin clearly rebounds. So I think I know what is really happening.

A striker-fired design, the P15 extends the firing pin when the trigger is pulled, and the firing pin retracts (little spring inside the big spring) as soon as the forces holding the pin out are overcome by the rebound spring. All in all a pretty clever design - pretty much the same as Glock and M&P.

But..

A soft primer, e.g. CCI500, can trap the firing pin stopping it from rebounding. The downward motion of the barrel unlocking drags the firing pin against the primer. This creates a teardrop primer indent and digs a bit of brass that is sucked into the striker channel when the firing pin retracts.

I'm pretty sure that Keltec design fix is to increase the tip radius of the firing pin. And/or increase the power of the little rebound spring.

I'm half tempted to load up some cartridges using CCI450 primers as they have a thicker cup. And I have some CCI Blazer brass cased factory ammo I can try for giggles.

Thanks for the input as I thrashed trying to really understand the issue!