.410 Contender
I have one of the .45Colt/.410 barrels (10"), and it does ok. I have never patterned it, (one of these days I will), but it seems to do better with the 2.5inch shells than the 3 inch. Might just be my limited experience though.
I have fired a few .410 slugs (remember to remove the "straightener" before shooting) and was impressed by the energy, not the accuracy, but again, never shot them on paper. I'm going to have to make a range trip to really check this out.
.45 Colt never shot well from the barrel, not nearly as good as my Ruger Blackhawk. I got a .45 Colt (only) barrel and accuracy was much better.
If you want a 20ga. "pistol", open up your wallet and get permission from the Feds, then search for the old Ithaca "Auto & Burglar" gun. This was a short barreled double barrel with a pistol grip, made in 20ga., and I think 12ga. as well. I'm sure somebody out there has one for sale, to a Fed licensed individual, for the right price. Or you could make one (Fed approval FIRST) from a cheap single shot or double shotgun.