tig weldind a patch onto the receiver won't hurt a thing. have the welder back up the inside of the receiver/patch with a copper block.

this does two things...first, it leaves a smoother surface on the inside of the steel which means less cleanup. secondly, the copper acts as a heat sink and helps minimize warping and distortion.

welding the brake on, if done correctly, will not harm the bbl.

tig = tungsten inert gas...generally argon used as a shielding gas. tig welders are usually automatically wire fed.

heliarc is also a gas shielded process, but most often is hand fed wire or stick filler.

a google search will fill you in on the fine points of each process.

the wildest welding i see is electron beam welding...in a vacuum chamber.