OK, good point. Looking them up, it's effectively a fairly simple hunk of metal. However, they are restricted parts. It reminds me of a story from a friend a

OK, good point. Looking them up, it's effectively a fairly simple hunk of metal. However, they are restricted parts. It reminds me of a story from a friend about how strange some laws about gun registration are in the US. He was telling me of one model of gun which became restricted, but existing instances were grandfathered in. To get around this, before the ban went in place companies manufactured "unmachined lower recievers" which were literally just hunks of aluminum or steel with serial numbers on them. However, because they are registered firearms those specific chunks of metal are heavily regulated and can't be taken on planes without being locked up for instance.