There is no ATF ruling about once a machinegun barrel it cannot be used for a semi auto.
The ATF has banned importation of barrels for rifles that are unimportable under section 925(d)
the ban originally allowed for their import as "replacement parts" which were not allowed to be used to build a new firearm.
They have since banned their import all together.
ban has nothing to do with complete rifles, although the rifles must be importable under 925(d).
The ATF has further pushed the boundaries, and does not allow for barrels that were previously assembled as something that would not be importable(if it was assembled on an unimportable rifle and then de-milled it cannot be imported as a loose barrel)
virgin barrels for vz58s would be allowed, except that they are under 16"
basically a barrel is considered for an unimportable rifle if it has a bayonet lug, or flash hider attached, or if it ever has, or if it was ever built on an otherwise unimportable rifle.
if the rifle is assembled outside of country, as long as the end result is importable it can come in.
