Dave Santurri, Gunsmith moved from RI to MA. He is located directly behind the building behind the Rat's shop. Dave told me that he will NOT be selling anything, only doing gunsmithing. Dave only applied for an FFL as a Gunsmith thru BATFE. He currently has a MA State License as a Gunsmith...
They do make them threaded, most are threaded. Is this for an existing rifle or did you literally end up with a spare M&P 10 barrel?
If you are trying to thread, pin, and weld, I would definitely not be going this route with an M&P 10 because of the various proprietary parts - one benefit of these rifles is S&W lifetime support, once you weld a break on there you lose that ability.
There is something different about an M&P 10 barrel extension and bolt, such that you can't just go changing to anyone's barrel without some real fancy footing (has been done, search around you will see examples)..
So in short you gotta buy an S&W barrel. One place online I just looked at sells their parts but a barrel isn't one of them. Maybe, probably unlikely, S&W will sell you one but I bet it's cost prohibitive if so - at least for the special contoured 18 inch..
If I was in a free state I'd get off that proprietary platform before I dumped $$ making a lifetime warranty moot, keep it for what it is or sell it, get an Aero M5 put together and modify to your heart's content.. or in a ban state similarly I'd be happy with it or would sell it and similarly get a platform that is flexible to modifying - ie if you had a DPMS based rifle you could have a few uppers for it...
With an M&P 10 you are kinda stuck, upside is you are backed by their service and they are good quality but downside is they are NFG for customization.