I’ll respond to this original question first:
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The NSG finish was anodized aluminum. There is a shop or two up in Michigan where the original SAGE chassis were anodized, and I think stripping and re-anodizing is the best coarse of action. Not inexpensive, but much better than Cerakoting, which can’t mimic the finish of actual anodizing.
Your black SAGE chassis is a later one that is not marked “Patent Pending” like the original Mod 0 chassis, so you’ll be cutting corners if you go down that route. The SEI Vortex is the later version with grooves and is different from the earlier ones that are smooth. So you have to decide what corners you are going to cut and what is your product budget? I probably wouldn’t try to hunt down an original (expensive) Mk 14 Mod 0 marked bolt - unless I had the proper “Pat Pending” NSG chassis to go with it.
This brings me to the larger challenge: Historically “accurate” Mk 14 Mod 0s are almost impossible to piece together at this point in time. Why? The unique “Pat Pending” chassis and some other parts like the bolts were made 20 years ago circa 2005. So original Mod 0 parts are long discontinued and most are used-up on replicas built out over time. I’m not saying it’s impossible, but as of 2025 it will extremely challenging unless you cut corners re key details.
If you started the project 6 or 10 years ago it would have been much easier and far less expensive, but circa 2025 it’s better to find a complete rifle for sale, IMO. This replica Mod 0 rifle was complete and probably a good price given the scarcity of the unique Mod 0 parts. Unfortunately it’s the only one that has come up for sale this year:
My opinion? Even w/ a budget matching the selling price of that rifle, you’re going to need a LOT of luck hunting down the “correct” parts, and it might take a long time…with no guarantee that you’ll find them.
IMO, what would be much easier and less expensive would be a nice replica of a U.S. Army EBR-RI (even with an 18” barrel). Your chassis would be perfect for that replica, and the parts used are easier to find…
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My buddy built this 3 years ago and it was relatively quick and affordable. He was a Marine, but decided replicating a USMC M39 EBR was just too hard and expensive, so he built a nice Army EBR-RI instead.