I've heard that the original Colts with gain twist rifling were capable of sub-1" groups on occasion, but I cannot verify that.
I know my very first group w/ my old ROA cut a not-very-ragged one-hole group at 20 yards, and it will do it pretty much on-demand when I'm having a good day. The load is .457" Speer RB, 45gr (yeah, I was surprised it held that much too) 2F Goex, and a greased felt wad under the ball.
I just picked up a stainless gun from about '81-'82 in the box with all the papers, apparently in unfired condition for $400. I nearly threw my shoulder out of socket reaching for my wallet. There is not much doubt in my opinion that it is the strongest, most reliable, and probably the most accurate C&B ever made.