Martini-Henri's, Springfield Trapdoor's, Snider conversions, Garands, Springfield 1903's, M1A rifles, Any and all Colt revolvers, CAR-15's, SP89's, HK91 and 93's, FALs, Blaser R93's, Beretta premium shotguns, MAC-10's, Winchester 1893, 1907, etc., 1911's, Browning High Powers, Walther pistols made during WWII, Luger pistols, Any and all Mauser rifles, S&W .44 Russians, etc. etc etc
My point is, almost any firearm can be used as an investment. It doesn't take long to find a firearm that will work.