All they were doing was silver soldering a piece of music wire to the bottom of the bolt breechface. Laid flat vertically it essentially becomes a firing pin, crushing the rim as the bolt closes. On the bottom, the wire pushes the cartridge out of the mag and into the chamber so no issues with the rising rim hitting an extended firing pin during feeding. However, the stock hammer fire setup was retained. When he loaded the first round he rode the bolt home, letting it close gently enough it did not set off the first round with the wire. After he pulled the trigger, firing the first round with the OEM setup hammer hits firing pin method, the bolt would cycle and now hitting the round full force when the bolt closes the wire essentially converting it to an open bolt fixed firing pin system. With no means of stopping it except running out of ammo or jamming. He apparently had one of the good AR7 and mag combos that was reliable feeding. That appears to have been what they did with all the .22s they converted, first shot by the oem method, then empty the mag as the bolt cycled full force.
Bolt bounce is not an issue normally with an open bolt fixed firing pin system as if it does bounce back it is after the round has already fired. With a hammer or striker system that drops the hammer or firing pin as the bolt closes can have problems with bolt bounce as it may close and then the firing pin is too far away from the cartridge as the bolt is bouncing back and the firing event happens during that microsecond of gap.
