Howdy
I don't know why R&D puts that limit for Black Powder in their cylinders because they can take a cowboy load of smokeless, which will generate more pressure than any amount of Black Powder you can stuff in the case.
I have two Remmies equiped with R&D cylinders, one is a fairly modern Uberti, the other is an old EuroArms Remmie I bought way back around 1975. Long before R&D cylinders existed. The Uberti is noticeably bigger than the old EuroArms gun. The EuroArms gun is more like the actual size of the original Remmies, the Uberti is a tad oversized. Actually, although the cylinder will take it, I don't much like shooting a 45 Colt case full of BP in my old EuroArms Remmie. The recoil is a bit uncomfortable. I can shoot that load in my Colts all day long, but it seems to kick a bit more in the Remmie, to the point of being uncomfortable. The other thing is, I don't much like stressing the frame of my old Remmie. If you think about it, when fired as a C&B gun, the Remmies don't see anywhere near as much pounding as they do with cartridges. A 44 cal ball only weighs around 150 or 160 grains, about as much as a 38 bullet. Bullet weight is a major component of recoil. A 200 or 250 grain bullet is going to develop more recoil and more pounding of the gun. With a percussion clyinder you will seldom put more than 30 grains of powder in the chamber. 30 grains of powder with a 160 grain ball generated much less recoil and pounding of the mechanism than a 250 grain bullet and 35 grains of powder. I particularly don't like stressing the portion of the frame where the loading lever pierces the frame. The cross section of metal is very thin there. While I'm sure the frame will stand up to a C&B load forever, I just think it's a different story with a 250 grain bullet generating a whole lot more recoil.
Which is exactly why Wild Bill Peterson and I invented the J/P 45-200 grain Big Lube bullet. I was looking for something lighter than the then available 250 grain PRS bullet, and Wild Bill wanted a bit less recoil in his pistols too. I had my R&D cylinders altered to accept the 45 Schofield cartridge, with its larger rim. Your cylinder may accept it without any alteration. My normal load for my old EuroArms Remmie is about 28 grains of FFg behind the J/P 45-200 bullet in a 45 Schofield case. Very mild load, I can shoot it all day.
You might want to try it.