.410's
There was a period of time in my youth where a .410 shotgun was all I had. My Dad, who was an avid rabbit hunter, also shot a .410 as his bunny gun a great deal as well. I've had more than a passing interest in the chambering ever since.
I am of the firm opinion that any shot size larger than #6 is TOO LARGE for the .410. Further, I can only advise #6 shot in the larger 3" shell. There is just not enough room in that tiny .410 shell for enough shot in the larger sizes to be effective. You see #5 and #4 shot in .410 for sale in the big stores all the time, and folks buy and shoot them. I believe this leads to the reputation of the .410 being a crippler. Dose a squirrel or rabbit with just a few #4 shot due to the sparse patterns because of low pellet count/large shot, and you may not collect the critter. Dad and I established early on that the 3" shell with #7-1/2 shot was by far the best killer on rabbits and I am going to extend that claim to squirrels as well. Another good load may be the 2-1/2" shell and #8 shot, but I do not have much experience with those.
The .410 is a difficult chambering for swing and shoot wing shooting or clay birds, but squirrel and rabbit hunting is a good bit different than shooting at flushing, feathered game. The "point and shoot process" of rabbits in front of slow dogs, or a squirrel on a stump is a far easier target than a grouse underfoot or a dove zipping past. On those semi stationary targets (bunnies and squirrels) the .410, when properly loaded, is very effective. Note that I am not talking about a squirrel 35 yds up, running from tree to tree in the canopy, but unalarmed squirrels on the ground, inside 30 yds.
All .410's I have handled and shot are fixed, full choke.