The .255 Rook Rifle ctg. (COTW) looks similar to the modern .25/20 Winchester. It is listed as small,rimmed, necked cartridge introduced by jJeffery and used in Rook-type rifles.

Small game and target number with 2 loadings, 9 grains of BP or 4gr. of smokeless - both with 65gr. lead solid or HP at about 1,200fps.

This round is listed as having a .255" bullet diameter (lead bullets to .257" would be just fine, if not perfect - depends on the chamber, of course).

The case is very, very close to the dimensions of the .25/20 Winchester etc. and undoubtedly, modern .25/20, .218 Bee or .32/20 brass is virtually perfect for this rifle's chamber - necked to take a .255" to .257" bullet, of course. It would surely be a fun gun to own and shoot and in that rifle, the ballistics could be improved upon some to make it even more fun as a small game iron sighted rifle.