I have the same Uberti, 1849 pocket .31 . You need #10 Remington caps, they work the best and fit perfect my 1849, Dragoon and Walker. Rem #11's work ok too, just aren't as long. I've tried CCI, RWS too but they fall off ( just pinch them first and they'll hold tight )
You can and should shoot #0 Buckshot in this gun. It's .320 and loads well.
.321 would be better, but is hard to find. Plus, 5 lbs of buckshot is about $15 and should be about 800 balls. Can' beat that price.
I punch felt, but some lube on top of the balls is fine too.