Re: It's been done before.
IIRC most tank guns now are smoothbore since things like finned and sabot rounds don't get much advantage from being spun upon firing, plus it's easier on the barrel. Not only that, tank guns are of a larger bore, making it easier to squeeze electronics into the shells. One of the tricks they designers of the AA proximity fuse had to deal with was making it compact enough to cram into a 20mm AA shell. Cramming course correction into a .50 round, particularly a rifled one that means it has to endure spinning, too, is another matter.