Blind pin & weld, which is why it is so widely the preferred method.  Virtually all the heat is localized to the muzzle device and not on the barrel itself, which is unlike silver soldier, which requires the barrel to be brought up to at least 1100F.  That's MUCH hotter than the barrel will ever get from the spot welding on a FS or brake.

Of course, the absolute best way is what Gem-Tech uses: a laser welder.  This keeps the heat extremely localized to just the area being welded.  But those things ain't cheap.

-Troy