The difference between an M-14 and an M1 from an NFA collectors perspective is about $30,000, assuming it's not a reweld. Speaking of rewelds, I have tried buying an uncut M-14 three times and failed all three times. They were all rewelds.
I've given up on buying an M-14.
For those not familiar with the situation, most existing transferable M-14s and M1s are rewelds. Here is an article on the issue.
https://www.fulton-armory.com/faqs/M1G-FAQs/Weld.htm
For those who don't want to read it, the last paragraph is the most relevant:
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'I have tried to tell dozens of "proud" owners that they have junk - the result usually is the same as calling the guy's wife names.
In two cases, the owners insisted on having receivers magnafluxed to prove me wrong; in both cases, I was right - they were rewelds.
There is one more point on the rewelds other than the ones you mention. If the receiver is shortened in the cut/weld process, the firing pin may be able to reach the primer before the bolt is locked, that is when it is not yet in the safety cutout in the receiver. The result could be nasty.
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It kind of upsets me when I see these guns in a museum. Assuming they're transferable guns, they'll never be shot and enjoyed. They're nothing more than paperweights and wall hangers. They'll never have a range day. They're in black hole, to never come out. Kind of sad.