I spent some of this Memorial Day weekend tinkering with the 8.6BLK Mosin. I bought a cheap takeoff barrel with the hopes of converting it to the "Mosage" system I'm developing so that I can also shoot cheap 54r through it.
My attempt at extending the existing threads did not go super well, in part because they were not coaxial with the outside of the barrel. So I TIG welded over the entire tenon with ER309 filler rod, my go-to for welding on alloy steels of which the exact chemical composition is unknown, and re cut the threads from scratch. Knowing what I know now, that will be my default procedure if I ever convert more barrels like that in the future. It came out great!
I also took the receiver stub the barrel came on, and made it a little thread gauge for checking them. I have and still prefer to use a thread micrometer, but it'll be handy for cursory inspection.