An M21 bringback for $400???? Run like heck.... I doubt it is legal as there were very few legal USGI M14s registered.
Believe the rifle and not the story behind it. Not sayin your friend is a liar as I know of someone who said the he brought one back through the mail piece by piece from Nam. I never saw it, nor did I believe he had the whole thing.
I've also seen several so called bringbacks only to fuind that they are what is referred to a rewelds. Some of those are legal such as Hahn Machine. He had permission to weld torch cut receivers back together. I have a copy of the letter ( the pearle letter) and it is contingent on the receiver having had been properly demilled.
That means torch cut with a quarter inch X cut through the sides and one cut from front to back down through the middle of the receiver.
That means 8 pieces welded back together.
The ones that I have seen welded by Hahn Machine were not demilled properly so I can't speak on their legality. They were simply bandsaw cut through the center of the sides.2 pieces.
Those that were welded by Mike Kelly (MKS) are all deemed to be illegal as Mr. Kelly used a copy of the Pearle Letter that was written to Hahn Machine as his approval. The ATF confiscated a large number of these. In some cases, the owners were allowed to remove everything and just give up the receiver, and in other cases, they lost the entire rifle. As far as I know, none of those receivers or rifles were ever returned to their owners.
The ATF has deemed the M14 Rifle as Once a Machine Gun, Always a Machine Gun.
Unless you are absolutely, without a doubt certain, that the rifle is clean and legal, run away.