Never hunted with a 20ga. myself, but know of others that have when I used to hunt in
New Jersey (shotgun only). Most, like myself, used 12ga., 2-3/4", and of course, this was before the rifled barrel came along. Anyway, those using the lighter 20ga. didn't seem to have any problem. And, for the ranges you're talking about (50yds. +/-), a
20ga slug will do the job.