I generally use a #6 shot and hunt cottontails. I use a modified choke tube for rabbits and it works pretty well. #4 shot is good for hunting heavy brush/cover. Some people use #7-1/2, but to me that's a little on the light side & a lot of pellets. Some run-of-the-mill #6 in 2-3/4" shell works well for most conditions. I usually buy the cheap stuff for bunny bustin' @ $5.99/box.

Oh yea... I'm using a 12ga Mossberg 500. But a 20ga will work great too. At least on cottontails. They are easy to kill.