can you see the internals of that weapon? you look at a fictional weapon in a video game and because it vaguely resembles a ppsh you tell me the weapon in the game is thus blowback operated? how do you know its not recoil?(gas would be hard with the space provided in it but thats possible too...)
and believe me i know firearms. its quite literally my job to know weapons. now, the largest cal ive heard of in a blow back pistol for example is 9mm markarov(technically 9.3m). off the top of my head i dont know of a blowback smg higher cal then a .45.... and there is the mk19 in the modern us arsenal that uses a high power version of the m203 grenade and is blow back. lol but the bolt on that thing weighs in exess of 20 pounds(god damn i hate working on that thing, and it isnt the weight).
yes you are right, unless you have an obnoxiously heavy bolt or a main/buffer spring so powerful a normal human couldnt charge the weapon you cant have a blowback operation for something as powerful as a 12 gauge. however that doesnt mean its too difficult to make a recoil system for the damn things. hell i remember no too far back seeing a couple recoil operated shotguns.... hell it could have a sort of gas operation like the benelli m4 super 90 shotgun, a system that doesnt require that much space for gas tubes and what not....