I've heard of guys using some .52 caliber minie balls for slugs in a 28 gauge...but you would have to do some good measurements to be sure.
I have loaded some round ball in 20 gauge, and have gotten a round ball mould to make some for 28, but I can't remember what the diameter is right off...maybe .515"?
I just got in my C-H dies for 28 gauge brass hulls.
I've had the .410 dies for while, but have yet to use them.
I remember reading where some some gentlemen with lots of time on their hands making some rifled barrels for 28 gauge and turning them into some real slug guns. However, this is useless in most southern localities, because deer shotguns have to be 20 gauge or larger.
It would be fun to play with though.
As for buckshot, I have two boxes of paper hull 28 gauge buckshot loaded by Eley. It is marked single ought, but I think that is in English sizing, not American, so it has to be smaller than ours. I keep meaning to take one of them apart and measure it to see what size it really is...likely equivalent to #3 Buck on the American measurements I'd guess.