The conversion cylinder have been on the market for WAY longer than the word "Cowboy" has been in use to describe a specific loading...
The conversions have always been rated for any standard 45 colt load shooting lead bullets at 1000fps or less. I have always thought this was hot and most standard 45Colt loads aren't loaded to 1000fps.
Bullets over 200gr just seem like too much for my Rem58 and They sure don't hit to the POA.
I find the best loads with my Pietta 1858 Remington with a 5 round Kirst Konverter is 200gr. xtp over a light book load of titetgroup or unique. I know it is a jacketed bullet, but it gives great preformance and accuracy, and it is still way within the pressure limits.
TrailBoss works well and so does Blackhorn209... both these powders are more spendy but they do prevent double charges.