My friend has a 3 band military Rolling Block. It has the standard verbiage on the upper tang with the last patent date being 1874. No other marks are visible on it except a "B" on the barrel immediately forward of the action. A chamber cast revealed that it has a case length of 2.58 with a neck of .461. The barrel at the chamber cast out to be .4405.  The numbers are similar to the Cartridges of the World's numbers for a 44-90 Remington Special except for its cartridge length: 2.44. The Sharps/Remington 44-90 has a case length of 2.63. So this one is right in the middle. It is not a 44-77 2 1/4 case or 43 Spanish case at all. The Cartridges of the World does not list the Remington 44-100 bottleneck so I don't know if it could be that. I don't see anything similar listed in the foreign military cartridges either.  The chamber will accept a new 44-77 cartridge from Buffalo Arms that I use for my Shiloh but the RB will not chamber it when it has been fired through the Shiloh. It seems strange to me that a military style RB would be chambered for something usually used in the Creedmoor RBs. Will it damage the chamber if I load a light 44-77 and use a pp bullet seated way out to bridge the gap between the end of the 2 1/4 case and the beginning of the barrel? Will that "ring" the chamber?

Have you tried chambering a 43 Spanish round or a fired/unfired case ? You can by a single piece of brass from BACO.

There is also a Spanish military cartridge called a Reformado.  My understanding is that .43 Spanish rifles chambered for bottlenecked cases sometimes were not accurate.  The Reformado is an almost straight-taper case.