One point that we have to remember is that those original cartridges used Boxer casings, made up of laminated foil and paper for the body. The rim was an iron washer and there were a pair of cups at the base, a coiled-paper innner base wad and the thing was basically held together by the primer. There wasn't a lot of room inside one.
When they came out with drawn-brass cases, a cardboard liner was used below the neck to reduce inside case capacity. This cardboard liner continued in Kynoch production to the end of Snider ammo production in about 1966. It's purpose was to reduce case capacity to approximate the capacity of the old Boxer case.
If you are using modern drawn-brass cases, your inside capacity will be MUCH greater than the original ammunition.
Try reducing the case capacity with a homemade cardboard liner (just rolled paper and glue), pop it in and THEN size the case. There should be enough of a "shoulder" to retain this liner almost indefinitely.
Snider Carbines are complete FUN to shoot. You can actually WATCH the bullet go if you have binos and someone else is shooting. The shooter, though, feels the Carbine TWIST to the right as the thing fires: all that lead taking so much time to get down that whole 20 inches of barrel!
But they are FUN!
BTW, you can make up brass from 24-bore brass cases. MAGTECH in Brazil makes these; Cabela's handles them in North America, 22.95 a box of 25.