I shoot a New England Firearms Ultra Slug Gun 20 Gauge. I usually use Winchester Gold Sabots and get 3 in. groups at 150 yards. Experimenting this fall with Hornady SST with the super sharp tip and I was getting 4 inch groups at 200 yards.
It doesn't have the knock down power of the 12 gauge but it still kills the deer effectively although a little more tracking may be necessary at times. I have been using the 20 gauge for three years and have taken four deer, two bucks, two does.
Smooth bore 20 gauges have a short range in my experience. Just