Quote:
During the bullet's travel down the bore between the gas port and the muzzle, we had a metered amount of gas fed to the action.
This gas does the following:
Upon reaching the gas key bolted to the top of the carrier, it turns down into the bolt carrier where it is given a nice place to expand.
This is the area inside the bolt carrier where the bolt lives.
Gas expanding here forces the bolt carrier back AND the bolt forward.
Note that the bolt is also being forced BACK by the gas pressure expanding the cartridge case on the other side of the bolt.
For a short moment in time, these forces are about equal.
Ideally, this is while the bolt lugs are unlocking and before the extractor starts pulling on the case.
The bolt carrier starts to move backwards against the inertia of the carrier's weight, the buffer's weight and the operating spring.
All of these effect timing, that's why we have different weights of carriers, standard, heavy (H), H2, H3 etc.
The next thing the carrier encounters are the cam surfaces against the cam pin.
Of course we know that the cam pin goes through the bolt.
Rearward movement of the bolt carrier causes the bolt to rotate.