Once again, it doesn't matter much whats on the barrel (flash suppressor, threads, etc.) but rather what type of firearm it goes on that is the issue. As an example, take a look at an MG42 or MG3 barrel - no flash suppressor, no threads, no bayo lug. Other than the attached barrel extension (which is what the bolt & trunion lock into), they are about as plain as a barrel can come. They are nevertheless prohibited from importation because the firearm for which they were designed is not a sporting firearm.