It ain't the shot size that determines recoil. its the weight.
1 oz of shot, buck shot, or slugs is going to kick the same if you are pushing it with the same amount of powder.
Let me start by saying I don't use shotguns for self defense. That's why I have my little revolver.
But I do load my shotgun shells, 1 1/8 oz at 1150 fps for clay targets and three gun where you use bird shot.
For slugs I use the 1 oz Lee mold. For buck shot I use a .319 cal round ball and stuff 9 in the hull. (a tiny bit over 1 oz, then pick a powder charge to get me about 1200 fps. I pick the wad that gives me the best pattern at 25 yards.
I use the slugs and buck shot for three gun events that call for slugs and buck shot. Also I don't have any sights on my 870 Police Shot gun. Cylinder bore 18 inches with only a bead. This was the same shotgun APD issued for carrying in patrol cars but I don't like shotguns for police work.
This set up works great for me.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071