Report Shortening gas tube & Tapering barrel to fit gas block: SBS conver in NFA Section Posted July 14, 2014 I match the ID of the gas block to the finished OD of the barrel plus .0005 just tight enough that I can drill the new pin locations and the gas block will not move. Almost, but not quite tight enough that I can take it off by hand. Still needs a light press fit for seal. Whenever you weld up a barrel or receivers you need to use a backer or a plug. I have machined my own rods/plugs to go inside the barrel (slip fit) when I weld and plates that I vise grip onto the receivers when they are welded. I use bronze for my backers as it is a dissimilar materiel and has better longevity than other backers. Jack What amperage and type of wire would you use to do that (Carbon steel wire)? Also do you cut the plugs at an angle because the ports are drilled at an angle? What type of metal is a typical barrel made of (Carbon steel?), is it lined? Any other words of wisdom or pics of your setup?