Where to begin?
A 44 caliber round ball over 25 grains of powder in a C&B cylinder will not develop the same pressure as a 45 Colt loaded with 25 grains of powder and a bullet weighing around 140 grains because of the greater bearing surface of the bullet against the bore as compared to a round ball. In addition, a bullet is mechanically crimped in place in a crimp groove, while the ball is only held in the chamber of a Cap & Ball revolver by friction. So couple both of these facts together and there will be more resistance to the bullet moving than the ball moving, which will result in higher pressure being developed in the cartridge.
Here is the pamphlet that came with one of my R&D cylinders that I use in one of my steel framed Remmies. It is pretty plain that they do not want their cylinders used in a brass framed revolver.
I only shoot my conversion cylinder Remmies with Black Powder, I don't think I have ever fired them with Smokeless. At least not that I can remember.
Regarding 'safe' Smokeless ammo to shoot in them, notice R&D recommends only 'Cowboy Ammunition'. The problem with that is there are no official SAAMI specs regarding Cowboy ammo. They just don't exist. Cowboy ammo is what ever the manufacturer chooses. I can tell you that most commercial Cowboy Ammo is loaded hotter than what I often hear at a CAS match. Some shooters load down their 45s to a level that would embarrass a 38. In addition, the 750 fps to 850 fps velocity stated on the pamphlet is also meaningless. Cylinders burst from too much pressure, not too much velocity. While velocity may be related to pressure, it is possible to achieve the same velocity with widely varying pressures, depending on the specific powder used, bullet weight, and the phase of the moon.
There you have it.
No conversion cylinders in brass frames for me, and if I was going to shoot Smokeless in them, it would not be barn burners. I would stay down near the minimum to mid level of pressure.