[alanint]

Correct, with a couple of clarifications. The semi is a blowback action. But the 1921 and 1928 where a DELAYED blowback action through the friction created by the Blish Lock. The bolts on the 1921 and 1928 are essentially two pieces, where the Blish actually mates the bolt to the actuating knob on the top of the gun. One could even argue that these two guns were of LOCKED breech, since the Blish Lock did have two ears which mated with channels in the receiver when the bolt drove home and were released when the bolt flew back. Something akin to a rudimentary roller lock setup like the MG34, MG42 and the HK series of weapons.

The M1 had a firing pin, spring and hammer, just like the 1921 and 28, but the bolt was simplified, made one piece and was actuated by a handle directly mated to the bolt and on the right side of the receiver, instead of the top. The bolt of the M1A1 was longer than the M1's and the firing pin on the M1A1 was machined directly into the bolt face and the independent firing pin, spring and hammer of the M1 were eliminated. In fact, this is the only feature which distinguishes an M1 from an M1A1.
The M1 bolt stopped when a channel in the bolt reached a battery position, allowing the hammer to drive home and hit the firing pin. The M1A1 bolt stopped when it could go no further than the cartridge base would allow. The M1A1 was slightly more suseptible to dirt and debris because of this, but had fewer parts to worry about than the M1. Both guns operated via a sear trip in the lower receiver, which would allow the bolt to keep going or to stop after each shot, depending on the position of the selector. Both full and semi auto fired from an open bolt.