I can understand that an under-powdered cartridge might send the operating rod only partially backwards, not enough to extract, but once in a while the rifle fires and that's it. No action at all resulting from gases or recoil.

I have the proper wrench, have removed the piston nut and piston, cleaned the port (which actually seemed clean, as well as the piston cylinder). The piston appeared in nice condition.

I figure one of two things are happening. One possibility is the piston is able to cock occasionally and "wedge", with no force transferred to the operating rod, although the tolerances seem tight enough to preclude that.

The other would be the bolt remaining in battery and locking up against any cam action.

This fired case remaining in chamber happens approximately once in a dozen or so rounds, and not at all at other times, so it is pretty random.

Actuating the operating rod manually at various (by hand) speeds does not result in any wrong results.

Not sure what to look for or how to test for it. Help!?