[Unclenick]

On thing it occurs to me to ask is about the condition of the extractor. If the hook has lost sharpness on the inside or the if the spring has taken a set and weakened, then the hook could slip off the rim under the higher force of extraction in actual firing, then snap right back over the rim on the return to battery. You could apply some Magic Marker to the rim of the case and the leading edge of the extractor to look for signs of that happening.

That said, if the slide is truly getting all the way to counterbattery, I would expect it to try to strip the next cartridge and jam in that case. But you don't have a filed hammer nose and I can't see how sharp the bottom of the firing pis stop is, so it's perfectly possible it cocks before reaching full counterbattery, allowing what I described to happen. You can test for this with it assembled. See if you can make the slide cock the hammer without picking up an empty case you put in the magazine.