My first deer was a button buck shot at 30 yards with a 20 gauge slug in 2 3/4". It was just a regular, smoothbore. The entry hole was little, the exit hole was huge. At reasonable ranges, a 20 gauge will do the job. I have a friend who's killed deer with a .410 and buckshot at close ranges, but I wouldn't recommend it (it's illegal in MD too).
-- B