1st puzzlement: There seemed to be some oddities in cartridge dimensions. Research has now revealed there were 2 .42 Russian cartridges: a rifle round sharing the same basic case with the .43 Spanish and .44-77. (I would in fact say that the .42 is the actual parent case) and a Gatling (Gorloff) round using the same bullet, but a longer and larger diameter case. (nearly 15mm longer!) Perhaps based on a lengthened Gras case. This would make sense as the Russians were doing a lot of work hand in glove with St Etienne at this time. I had always though the Gatling round was the same as the rifle, but evidently not.
It also appears that Buffalo Arms may have mixed the dimensions of these two very different cases! I won't know for sure until I get the rifle in hand, but I believe .43 Spanish brass will be the way to go here.
(Update: yep, "Cartridges of the World" first mixed up the case dimensions long ago, and it has carried over to the present day. BTW, Gorloff had NOTHING to do with the rifle cartridge, the Gatling cartridge was his.)