.410
At present, my only use of the .410 is as a pest gun around our place. A Winchester 37 resides along the side door to dispatch snakes, rats, pigeons, or other vermin as necessary. But....
I have a lot of history with the .410 as a rabbit gun back in my youth. Both my Dad and I used .410's , along with his beagles, to kill a LOT of bunnies. Our guns were full choked, one was a Win 37, later a Mossberg bolt, then a Savage bolt (both of those cheap bolt shotguns "broke", we hunted and shot a lot) and finally a Savage 67 pump, and a High Standard pump. As an adult, I went and bought a Remington Express 870/.410, and still have it, but I do not run any beagles at present.....but I do not intend to sell the little shotgun.
For rabbits, I think without question, the best shell is the 3" version with
#7-1/2 shot. As most all know, the .410 has a small payload. This handicap is increased when folks shoot heavier shot, and by this I mean ANY shot larger than # 7-1/2. And often, when you find shells at Wally World or other large venues, what is most common is #6, or heaven forbid, #4. Such larger pellets lend to very thin, crippling patterns at all but the shortest ranges. But #7-1/2 from most FULL choke guns, shot will throw killing patterns for rabbits out to 25-30 yds. Most of our bunny shooting was way inside that, and the .410 so loaded worked just fine, like in .....ideal or perfect.
Too, though .410 shells are expensive, comparatively, you can carry twice as many afield at an equal weight. That can matter on a good bunny patch with lots of shooting, far from the truck. Also, even at short ranges, inside 20 yds, the .410 will not hamburger a rabbit like the bigger gauges will.
I'll add, that shot smaller than #7-1/2 may be too small for use on rabbits and squirrels. I have a quantity of the 2.5" shells with #8 shot, but have never used them on critters. I suspect the small pellets may run out of energy quickly and will not punch through hide and muscle of fur targets (as opposed to feathered targets) at all but really close range.
Finally, all my .410 shooting at game has been for cottontails in front of dogs. Shots were close, usually at bunnies moving very slowly, sometimes even stopping, in front of a patient dog or two. I have no experience on wingshooting with the .410, other than a few unlucky quail, and a couple of pheasants that got up close while rabbit hunting.