Even a 20ga will get it done at 50yds, if you do your part. I took a small buck 3 yrs ago at about 60 yds with my Tracker I and Remington Sluggers. It wasn't a "Bang-Flop" because my shot placement wasn't quite right, but it didn't run because I severed its spine. Your 12ga will do just fine, provided you do your part.
Good hunting.
cookiemann