Most of the small game I go after are shot with rimfire or centerfire, but I found a great deal on some small game loads for my 12ga last fall so I figured I'd give them a shot. I think they should do just fine for rabbits:
My shotgunning of rabbits has always been behind a pack of beagles or kicking them out of brush piles. That meant quick moving small targets that were running and dodging in thick cover. Usually fairly close ranges. A 20 gauge and improved cylinder or modified choke with an ounce of #6's was about perfect. My dad preferred the 16 gauge choked the same. Pop was murder on rabbits with his old Belgian Sweet Sixteen Auto Five. Our rabbits were cottontails. A western hunter shooting jacks at longer ranges should benefit from a 12 gauge with some more choke and larger shot.
For jacks I use 1 1/4 oz of #5s from a 12 ga. I never change my light modified choke for anything. More choke might result in more kills when they are sitting at long distance, but a disadvantage on the close range runners.
I take tons of jacks while after cottontail, chukar, and quail. When not going after them specifically, I use 1 oz of #6 in my 20 ga with a LM choke. This load will still drop jacks without a problem, but #5s from the 12ga drops them with noticeably more authority when they get out a little further or the hit is marginal. Overall, I prefer rifles if only after jacks just for the Challenge and practice.
That is some great info... thanks. I don't currently have a light modified choke, only SF, F, M, and IC. Next time I'm out patterning I'll test #4s with modified choke at 20-25yd and 40yd, so I have some comparison with my #4s and full choke.