Read my post, "Proper Use of Cap and Ball Revolver" above. It will answer many questions.
That said, let me reiterate or add:
I had a brass-framed reproduction of the 1851 Colt, in the unauthentic .44 caliber, in the early 1970s.
On three separate occasions I experienced multiple ignition with it.In those days I used a .451" ball, Remington caps, DuPont FFFG black powder and Crisco slathered over each ball.
I discount the use of lubricant over the ball as a remedy for multiple ignitions. I used to believe it, but no longer do. I believe that multiple ignitions begin at the rear of the cylinder, when flame gets under a loose-fitting cap, or a loose cap falls off during handling or from recoil.
Since about 1975, I've used hard felt, made of wool, soaked in lubricant and seated on the powder under the ball. Long ago, I'd scrounge for old hats as a source of wool felt. Today, I buy sheets of hard, 100 percent wool felt from Durofelt. The price is right, I can make thousands of wads, and shipping is free within the continental U.S.
For nearly 40 years I've also pinched the caps into an elliptical shape, so they have a springiness that keeps them on the nipple when seated. Yes, I use properly fitting nipples but I still pinch the caps into a slight elliptical. I believe in it; old habits die hard I guess.
I also balls of .454 or 457 inch. If the revolver is properly made, this is no undue strain on the rammer. If it's a piece of junk, made by gunsmith apprentices on a Monday, it may result in damage.
I would not buy a brass-framed revolver. More often than not, they are of poor quality.
Yes, there are poorly made steel-framed revolvers out there, but brass-framed ones outweigh them. Having a brass-framed revolver means you should not load it to its full potential, because such heavy loads cause premature wear and tear.
Spend the extra $20 to $60 and get a steel-framed revolver.
If money is no object, get a Remington-patterned .44 made of stainless steel. Shooters new to cap and ball revolvers tend to avoid the cleaning required, or fail to do it well. Stainless steel will rust eventually, but it's more forgiving.
I would go with the .44 because oversized balls for it are readily availble: .454 or, if you can't find those, .457 inch.