A small diameter 2% thoriated tungsten TIG electrode sharpened to a needle point and extended out a bit further than normal from a very small diameter ceramic cup can fit into a very small place, along with the filler rod. The ribs would of course be removed during a sleeving job, but the welding is done so close to the breech that spreading the muzzles apart wouldn't help much at all. I have no familiarity with laser welding, but I wouldn't be surprised to hear that it could be done in even tighter spots than TIG.

Voting for anti-gun Democrats is dumber than giving treats to a dog that shits on a Persian Rug