I had Swedes long ago when I collected Mausers but since they were the only non german Mausers, I sold them away, and later I sold the Mausers.
Most of the collection that is detained only because of my professional status can't be owned by individuals in US because of NFA, only corporations or federal organisations could import them. Here the once a MG , always a MG does not exist. It can be converted to semi auto, or deactivatezd. The local deactivation keeps the weapons intact except removed ejector, ground bolt face and plugged barrel. A few pieces have already been sold to collectors after deactivation. Some other countries allow at various degrees individuals to own NFA classified items like live select fire weapons for shooting and/or collection purpose to certain extent: Belgium, Luxembourg, Finland, Germany , Switzerland...
Edited to add: I forgot Malta, the law changed recently, allowing collection of select fire weapons made before 1946.