I have the 450 Bushmaster and love it. What JonM says about big, slow bullets is right on from what I've seen. Out to 200 yards these things kill! I don't know about available .50 cal. bullets but the 450 Bushmaster uses bullets made for muzzle loaders and you can get heavier bullets that are plenty strong enough for the biggest game on this Continent. Guys have killed grizzly's with Hornady factory loads with one shot.
As far as being NYS legal just be sure it doesn't have a threaded barrel and if it does that a cap or compensator (NOT a flash suppressor) is pinned and the pin welded or the comp it's self is welded permanently on the barrel. If the mags are modified to hold no more than 10 rounds, you should be OK.
Enjoy your big bore!