What supports the round against the impact of the closing bolt/firing pin before it headspaces against the chamber mouth?
It is a very small amount of distance, but because of the construction of the bolt and barrel, the bolt is still moving forward when the round stops against the chamber mouth. Its that last little bit of bolt movement that fires the round.
..assume that the hammer in a 1928 or M1 Thompson has a similar effect. That drives the separate firing pin faster than the bolt is pushing the round and pops the primer against inertia of the round.
The "hammer" of the 1928 Thompson is a simple pivot lever that strikes the frame of the gun, as the bolt shuts, and pivots, the other end transferring force to the firing pin.
The semi auto Thompson is a completely different gun inside. In order to obtain BATF approval, the entire inside mechanism of the gun was redesigned, and is approx. 1/4" "off in dimension from the SMG, so one cannot simply put SMG parts in the semi and convert it.
This resulted in a gun that needs almost gorilla strength to cock the bolt, compared to the easy "finger cocking" of the SMG. All hail and thanks to our wonderous Treasury Dept!
As to the M60 op rod, I'm sorry, but in the shop, we never got a round count on the small arms. Mortars and Recoilless Rifles had logbooks, with round counts, but not the small arms. I can tell you the time period was 75-78, and about half that time I was in Europe, so the round count on the guns was from training, not combat, generally.
I can tell you that we tried not to replace the op rod, unless there wasn't another choice. Many times, we would just stone the op rod back to smoothness and send the gun back out. And, it wasn't the sear engagement that gave the trouble. It was the back of the upright portion of the op rod, where the bolt is cammed open and shut. The bolts would tear up the op rod at this point. Bad design? or bad execution? I can't say, but I can say that it happened, and happened regularly. NO gun, particularly an automatic should chew itself up like those did.
I understand someone finally got a clue and changed the cam angles to reduce this occurrence, I hope that's not just an urban legend, as it really needed done.