I second the H&R Ultra Slug in 20 gauge!! This will be my 4th or 5th season hunting with this gun and I agree.....It REALLY does the trick. The gun is fairly inexpensive and it is a pleasure to sight in. I've been able to re-load quickly and bag more than one doe at a time on several occasions so the one shot does not bother me. Good luck.
I've got a Benelli super 90 with a Bushnell 1.5-9 power scope, works great, though if I had it all to do over I would probably go with one of the H&R single shots. I have never had to shoot a deer more than once with a slug.