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From the BATF:
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.
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