ATF never gives responses with that much detail.  Instead, to get a writtin response, they generally require you to submit a sample, and they will tell you if it passes or not, sometimes with a small amount of additional information.  If this seems incredibly unhelpful and frustrating, you're right, it is.

Generally, a blind hole is drilled (one that doesn't go all the way through) into the brake such that about half of the diameter of the hole goes through the muzzle threads.  A steel pin is inserted and then welded over.  Though not legally required, most companies will turn down the excess weld and refinish for cosmetic reasons.

This is the preferred method, as it allows you to keep all welding (and the heat associated with it) concentrated on the muzzle extension and not the barrel itself.

Other approved methods:

- 1100F silver soldier
- >180 degree circumference weld
- 4 equidistant tack welds

-Troy