Theory... possibly a shaved piece of lead from the wadcutter bullet stuck between the barrel and cylinder, wedging the cylinder ratchet against the hand? I know that if the hand can't freely move that the trigger won't return. Or possibly a shaving getting in the slot for the hand itself? (Improbable, but not impossible.) I am of the belief that it was the ammo and not the gun if it works fine with other types of ammo.