As long as the lathe has at least a 1.5" hole through spindle bore everything else will be good enough for gunsmithing. Condition being the 2nd criteria to for.

For home gunsmithing a 12"x36" or 14"x40" single phase 220volt (120v is also good enough on a 12x36) is ideal. Again as long as they have a 1.5" spindle bore hole.

The 10"xxx lathes have a 1" spindle bore and some 12" also have a 1" but I've seen many 12" with 1.5" bores. Just about all modern 14" lathes have a 1.5" spindle bore.