The Ruger Old Army, being that it is not a reproduction, cannot be used in International muzzleloading competition. Given it's bulk, weight, and high sight line over the grip, that may not be a big deal, though, as it still wouldn't be ideal.
Two of the best reproduction cap & ball revolvers suitable for high level match shooting were the Hege "Match Maximum" (a very nice Remington 1858 repro) and the Feinwerkbau "History #2" (a very nice Rogers & Spencer repro). Both were made in Germany until recent years.
Also, Andy Baumkircher built some excellent handmade copies of a variety of cap & ball revolvers. He is retired now, but his guns are very sought after.
Pedersoli makes a very nice copy of the Remington 1858 built for competition and fully compliant with MLAIC rules. It is probably now the preferred choice around the world for match competition use.
My old buddy Mario Cruz, who lived in Portugal, posted this video shooting the Pedersoli in practice.
https://youtu.be/uZ8EYQ9ng30
And, "Cap & Ball" in Hungary made this informative video about the Pedersoli, including load info, bore and chamber dimensions, etc. that I think many of you guys would enjoy:
https://youtu.be/M3cHmmjct8U