Maybe non-bolt action shotguns can't handle the chamber pressure to really push one to it's full potential (120mm APFSDS MV is 5200+ fps, and a tank has one he11 of a breech), but also the really long, sharp design to help get the velocity wouldn't expand in most game, and many states ban non-expanding ammunition in large-game hunting. If they could get past the chamber pressure (if it even is a problem) an APFSDS 12ga round would be great for defeating body armor at extended range, and should be extremely accurate (I would guess 2 MOA out to 300 yds should be easy). It could make the M16/AR15 obsolete as a LE or homestead defense firearm. Come to think of it, an all-brass shell with a consummable top instead of a crimp would help contain the pressure. Then again, probably nobody would develop and market a new round made specifically to defeat body armor (see what happened to the Black Talon just because it had a black tip). Still, you can get stuff like flechette, bolo (the two balls connected by wire), and even payload slugs you can fill with powder and fuse with a shotshell primer, so I don't see why someone hasn't made one yet, even just for the novelty of it.