Ok,
I’m not the expert - just another tinker with the platform.
Majority of 9mm are blowback - probably 99%, with the cartridge housed in the bolt / carrier - it’s one unit with the cartridge seat milled into the face of the carrier. The carrier is slotted for a static ejector that is fixed into the lower or mag well adaptor - it’s the dog leg looking piece in the left of the lower - looking from the rear.
There is no separate “bolt” as with the stock AR - the carrier becomes the “bolt” but you’ll see the terms interchanged. The 9mm one piece is typically heavier than a stock BCG - it’s a solid piece of steel.
CMMG designed a “radial delayed blowback” bolt with a matching barrel extension. The bolt and carrier look stock, but the assembly is spring loaded and pinned together - and there is no slot for the ejector - the bolt has an ejector in it. If you put the CMMG upper on a 9mm lower, the fixed ejector has to be removed - typically just knocking out a roll pin.
Basically- the radial cuts on the bolt lugs and matching barrel extension cause the carrier to move to unlock the bolt, so the blowback pulse is somewhat lesser than a straight blowback pulse.
Neither recoil is bad - it’s a 9mm, but it’s typically more than you’d expect in a small pistol cartridge in a rifle. I’d compare the difference to carbine and rifle length gas tubes - the recoil from both is mild, but the rifle length is typically slightly less.
Buffers - Stock is no marks, and you go up from there - H1, H2,H3 - you can Google the weights - I can’t remember them off of the cuff. If you’re over gassed (edited) in a direct impingement rifle, a heavier buffer may slow the carrier down enough to balance the system.
If you’re a shotgunner, it’s similar to switching your gas gun between field and trap loads.
Thank you for the info.
