What brother did,he bought a new bolt from Armalite and sent it to Krieger.He ordered the 20 in NM contour,fluted.It is a post-ban config,no threads,no pickle,just a crown.The gas port is drilled,and Krieger makes sure it is drilled into a groove.
Brother does have the reciever clamp fixture,bbl nut wrench,etc.He downloaded Armalites installation/torquing instructions.
Now,fair to say,it is not cheap.It was also approx 90 days.
As far as welded on pickles,I have been leary of them.Seems like some sort of distortion at the crown is likely as the weld solidifies and shrinks.Thats just my speculation,it may work fine.
To stay legal,you would not want to shorten a 16 in,but,if its a 20 in,or longer,cutting the threads plus whatever you think might be the heat affected zone,plus a recrown,is a reasonable thing to do.I have shortened two 24 in bull bbls to 20 in,gas system worked fine,and they are easier to shoot offhand.