CCI #11 are loose on my Pietta 1860. A little pinch keeps them on, otherwise they fly off after the first shot.
CCI #10 fit perfectly.
Your Uberti may be the opposite. Also, from what I understand, Remington #11 is not the same size as CCI #11. In other words, all #11s are not created equal.
You may just have to try some out. Over size is workable, just pinch them. Undersized, well, you'd have to cut them to get them to fit.
I don't use a capper, just put 'em on with my finger. Capper might be nice if it worked well; sometimes those little buggers are hard to get on without dropping them a couple of times.