This reminds me of an idea I had from watching video games – this is almost identical to the Super Shotgun in Quake 2.
However you can make one easily that would also be semi-automatic. But as it is with all double barrel shotguns I think that the ATF considers any gun that fires more than 1 shot per pull of the trigger to be a machinegun. Even if it fires both of them at the same time. Thats why this pump action only fires one at a time even though every pump loads 2.
Take two Saiga-12 Shotguns, a Lefty and Righty, join their charging handles and trigger together into one. Now you have a semi-automatic shotgun that fires both barrels every time the trigger is pulled. Put muzzle breaks on both barrels and a nice rubber pad on the butt and it wont even break your shoulder.
The only other way you’re going to get that kinda firepower is with a custom super caliber load like an 8 Gauge or 4 Gauge (hey, a 4 gauge works out to be 25mm, so a 25mm grenade launcher is already there).