The Remington RB pistol for the Navy was chambered for the 50 cal. Navy, aka 50 Springfield which was the original version of the 56-50. It was later chambered for the 50-45, which was a longer cartridge chambered and developed for the Remington RB Carbine adopted by the Navy. This simplified ammunition for the Navy. The Army version of the Remington RB was chambered for a shorter round, the 50 Army, which is a more well known round. I see a few 50 Army cartridges at shows, but rarely a 50 Cal. Navy.
At one time I owned a 50 Cal. Navy RB which worked with the 56-50 Spencer-- recoil was pretty stiff, and I imagine would have been pretty rough on someone with smaller hands. It was certainly a stomper, though. about like a 45 Colt on steroids.