I have done quite a bit of experimentation with handgun shotshells. The gentlemen who point out their limitations are correct, almost without exception. Seventy-five foot lethality is a lot to ask.
I believe bore size is critical. Bigger bores allow more shot, bigger shot, and encourage better patterns. As has been pointed out, rifled barrels are terrible choices for shot loads, so again, expectations should be moderated.
I have used a lot of Speer shot capsules, and they work well. Years ago, though, I moved on to gas checks. I seat one, cup up, over the powder charge, then fill with shot, and then load another check cup-down to seal the works. This can require opening the case mouth more than is normally prudent, and perhaps more crimp as well, but I consider these to be "special" products and don't mind donating a few cases to the effort.
Over the past year or so I have been experimenting with brass shotshells (in 12 gauge) with blackpowder and traditional methods. I have applied some of those techniques to handgun shotshells. My latest bit of foolishness is to load between a quarter to a half case full of Trail Boss, then seat a cork wad (these normally need to be punched by hand, although some of the appropriate sizes are available from muzzle loading supply houses like Track of the Wolf), then fill with shot, and finally seal with a card wad or two - again either hand punched or luckily found on line. The card wads can be crimped into place and/or sealed with Duco cement, which is flammable and leaves no trace in the gun. Early results are better than anything I've yet tried, but I don't yet have enough experience with them to give firm reccomendations.
TL;DR: Use the biggest bore you've got. Speer capsules are fine. Gas checks are cheaper and better. And traditional blackpowder shotgun techniques may be the best-performing option. And get close!