First try: it indeed fully seals between the two O rings, even just by pushing it in by hand. There is an air cushion of about 6-7mm. This would break the piston or the gear on the second shot.
I removed the small O ring and there is a noticeable resistance when pushing it in by hand and covering the nozzle, but that didn't make much difference to the impact sound. The gearbox cycles fine like that though.
I'll check with smaller O rings that won't seal perfectly.