Great info in this web page, here is one link:
[url]http://www.cs.cmu.edu/afs/cs/usr/wbardwel/public/nfalist/atf_letter40.txt[/url]
DEPARTMENT OF THE TREASURY
Bureau of Alcohol, Tobacco and Firearms
Washington, D.C. 20226
JUN 18 1998
Dear Mr. :
This refers to your letter of March 31, 1998, in which you ask about permanently attaching a muzzle device to various firearms.
A muzzle device, such as a muzzle brake or barrel extension, which is attached to a barrel by means of welding or high temperature
silver solder having a melting point of at least 1,100 degrees Fahrenheit, is considered to be part of the barrel for purposes of
measurement. A seam weld extending at least one-half the circumference of the barrel or four equidistant tack welds around
the circumference of the barrel are adequate for this purpose.
A firearm having a muzzle brake, cap, or barrel extension permanently attached by those same methods to cover the threads on a barrel, would not be considered to have a threaded muzzle.
Please note, however, that any muzzle device or barrel extension which functions as a flash suppressor or grenade launcher would
still constitute one of the qualifying features of a semiautomatic assault weapon as that term is defined in 18 U.S.C. section
921(a)(30(B). Industrial adhesive products are not an acceptable method for permanently attaching a muzzle device.
We trust that the foregoing has been responsive to your inquiry.
If you have further questions concerning this matter, please contact us.
Sincerely yours,
Edward M. Owen, Jr.
Chief, Firearms Technology Branch