I used this procedure to shorten a Bushmaster 16" barrel and pin on a Phantom flash hider.
1. Stripped and shortened the barrel with a bandsaw, then recrowned barrel in lathe.
2. Turned down and threaded barrel. Cold blued the bare metal. Reinstalled front sight and barrel nut.
3. Installed and indexed Phantom using a peel washer.
4. Filed off Parkerizing on bottom of flash hider and drilled a .124" hole through the Phantom and into the barrel at least .125".
5. Used a steel pin that I left just a bit long, and using a TIG welder, made a rosette weld out of it, fusing the pin and flash hider.
6. Used AlumaHyde II, touched up the bottom of the Phantom.
Overall length is about 16.25 inches.
Now, along comes my idiot brother-in-law who claims I need to use three spotwelds between flash hider and barrel.
What's the scoop?
You're good. Blind pinning and welding of a FH to meet NFA length limits is an ATF approved method of "permanently" attaching a FH.
I used to think the jokes about brother-in-laws were just jokes. Then I got married.