Wha?
Are you serious or delirious?
The 12 gauge is the epitome of limitless. You can bag everything from the simple quail to an elephant or even a wooly mammoth with a 12 gauge and the right ammo.
Well, you're not gonna whack a deer at 300 yards with one.
I think Kurt's right...a Bush length M1A or an AR10 is probably about as good as it's gonna get.