.410's
My Dad was an avid bunny hunter,and my best times with him were with his two good beagles, Jack and Jake on a cold fall morning! My Dad and I killed a bunch of rabbits back in the day, and we both hunted with .410's of various makes and actions depending on the year. We found that the various econ bolt .410's did not hold up to all the shooting we were doing, and basically wore out/broke a Mossberg bolt and a Savage as well.
We ended up with a High Standard pump, and a Savage/Springfield 67 pump. Not top shelf guns, blue collar all the way, but they worked and held up well.
What stayed the same was the shell and the choke. Three inch, #7-1/2 shot, and all those field guns were full choke. Most shots were close enough that the other loads posted, with #4 and #5 shot would have worked just as well. but past 15-20 yds, the pattern from the big pellet loads were pretty thin if you took a shot at a running bunny. We thought so anyhow.
Dad was a 16ga fan too, and his "heavy" rabbit gun was Savage double in 16ga, then later another blue collar Springfield pump in 16. Seems like the 16 ga got #6 shot.