Quoted:
How hard would it be for a talented welder and machinist to reweld a cut reciever, using the correct jig, so that the gun functions properly and safely? Is this something only someone with experience building firearms should attempt?
ETA: this would be a semi-auto build firing from a closed bolt
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How hard? Demil specifications require 1/4" kerf cuts in most instances, meaning you've not only got to get everything lined up correctly, and remove any features which would make it a machinegun receiver when finished, but you've got to trim the stubs and weld in machined repair sections, then do final finish machining to dress the welded areas.
It's been done, many times, but it's not trivial. Also in the case of a load-bearing receiver, like the M14, the welded areas aren't as strong as the forged original was. Less of a problem when rewelding a non load-bearing receiver like an M16.