Complex sentence: These replicas are very popular in civil war reenacting and Cowboy Action Shooting, either as cartridge conversions or shooting black powder or modern substitutes, just as the original Remington cartridge conversions were popular on the actual Western frontier of the 1860s and 1870s.
Split into simpler sentences: These replicas are very popular in civil war reenacting and Cowboy Action Shooting. These conversions, of course, being akin to the original Remington cartridge conversions used on the Western frontier of the 1860s and 1870s.