The Kirst and R&D cylinders are made of 4140 steel, they are rated for smokeless. As thin as the Remington 45 cal cylinder gets at the web, standard Cowboy loads are as strong as you want to get, they are not Rugers. The reason both makers warn against using them in brass frames is not blow-up, but the stretching of the frame, and in the Colt, the stress on the arbor screwed into the brass. A heavier, and usually harder bullet than the roundball hitting the forcing cone is trying to take the barrel with it.
I have shot Kirst and R&D converted Uberti and Pietta Remingtons and Colts for years in 38 and 45 with smokeless and BP and had no problems with the guns beyond the more rapid wear the cap & ball mechanical parts undergo compared to the harder internals of the cartridge revolvers by the same makers.
The same safety, of course cannot be expected if the cylinder is gunsmith converted using the original cap & ball cylinder. No smokeless allowed there.
As Das Jaeger said, the case can't hold enough black powder or Pyrodex to blow up a Kirst or R&D cylinder.
Actually, 777, though hotter, is closer to the original black powder loads, as the originals were in balloon head cases with more capacity than today's solid head cases, you cannot get 40 gr of black powder in a 45 Colt or 44-40 in a modern made case with a full weight bullet. A full load of 777 in a 45 Colt case is a formidable load, the hand holding the gun certainly knows it went off. I shot it in a converted Ruger Old Army, but hesitate to shoot it in a Remington.
