There were allegations of this a couple years back, but ATF is now charging someone with it:
The defendants accomplished this by “harvesting” serial numbers from older machine guns, which would be destroyed, and then welding those serial numbers onto larger, more expensive machine guns that they had manufactured. By so doing, they allegedly evaded the 1986 federal machine gun ban, which prohibited civilians from possessing or transferring machine guns manufactured before May 19, 1986. The indictment says this pattern of conduct involved 34 machine guns, allowing the defendants “to profit from the possession, transfer, and sale of machine guns that the defendants and others were otherwise prohibited to manufacture, possess, transfer, and sell.”
Stamp a registered MAC serial number onto an AR receiver = profit. ATF’s horrid lack of management of the NFRTR database contributed to this scam. And if I were the guys involved, I’d point that out at trial.