As for reproductions, although by no means an expert I would agree that you are best off sticking with .56-.50 … less problematic than those which have been "tweaked" to use pistol-caliber cartridges …
Admittedly, I have not subjected my reproduction .56-.50 Model 1865 Spencer full length rifle to any torture tests, but it functions very well for me as long as I keep cartridge OAL within the necessary parameters -
As a Canadian military reenactor/shooter, such a rifle was very high on my "must have" list until I finally acquired it a some years ago - of the limited quantity of only about 3,000 Model 1865 Spencer full-length rifles manufactured, fully 2,000 came to Canada for our Militia. (The chap above is a Rifleman of the Queen's Own Rifles of Toronto … a unit I portray …)