The reason shotguns had limited use is simple-- ammunition supply. You are now adding another cartridge that needs to be produced, stored, and delivered. The Germans had a hard time supplying just 9mm P and and 8mm M to infantry units. Introduction of 7.92mm K was a nightmare. You seriously think they could add 12G? Even today it's a supply nightmare. I carried an M4/M203 and used 12G adapters with breaching rounds in the 203. Those adapter rounds had to be kept totally separate from my normal 40mm rounds. That way I would never unintentionally grab the wrong round in battle or at night. It also made it easier for my team to grab the right ammo in case I was a casualty. Now the Army is issuing the LSS in lieu of the M203 for breaching ops. The last manual I read for it stated that it was for breaching only.