Cascadian, NATNWO & Libertopia state firearms:
Becuase these factions often use firearms that are rare &/or exotic in real life, & becuase one can build replicas by other means, I'm going to reccomend using the closest models of airsoft guns available.
The allaby for using these would partially be substitute to real story guns, partially that in an alternate timeline Cascadia just copied weapons.
1870's-1896: Sharps rifle (up to 1877), Winchester lever action rifles.
1897-late 1930's: Lee-Enfield rifle. Since finding airsoft Lee-Enfields would probably be hard, M1903A1 Springfield or later models would be acceptable.
http://www.leokoehler.com/index.html