The BATF considers a permanently attached barrel extension to be welded or brazed on, not soft-soldered.
Yes, I know lots of guys make legal TC Carbine-length barrels by soft-soldering on muzzle brakes, but they really don't meet the Federal requirement because they are easily removed.
Yes, a propane torch and lead-tin solder can be used to attach that muzzle brake. Thread barrel to fit brake or turn to make slip fit. You need a lathe to do this. Make sure that all bluing, oil, and dirt are cleaned off first and use plenty of flux. Attch brake and clamp in alignment while heating BOTH with propane. Hold barrel in vertical positio to prevent flux and solder from spreading to blued barrel. Acid flux will remove bluing. I used rosin (electrical use) flux. Apply solder and allow it to wick into the joint Cool and clean off flux with alcohol and brass bristle bruss.
