Actually, those conversion cylinders are perfectly happy with modern smokeless powders suitable for their caliber. Though it’s more about velocities than pressure. I can load any smokeless .45 Colt for my cylinder that is listed for the standard pressures, (about 11,000psi). But it is highly recommended to keep your velocities no more than 850fps, (the original velocity for the Colt SAA). Frame stretching is the issue. Those cap and ball revolvers are made from a bit softer metal than modern revolvers. Remington’s are stronger than the open top Colts.