Revenue Ruling 55-570 A gunsmith desire to affix a sleeve-type muzzle brake to the muzzle of a short barreled rifle (a firearm within the purview of the National Firearms Act) in order to lengthen the barrel of the weapon to 18 inches or more, thereby removing it from the classification of a firearm as defined in section 5848 of the Internal Revenue Code of 1954. Held, after proper registration of a short barreled rifle, the barrel may be lengthened to 18 inches (or more) by the attachment of an extension, such as a muzzle brake or barrel band, by silver soldering or other permanent means. After the barrel has been lengthened by approved means, subsequent transfer thereof may be effected without incurring liability for the transfer tax imposed under section 5811 of Internal Revenue Code of 1954. [note that the NFA defined a short rifle as one with a barrel under 18 inches, not the current 16, until around 1960. The point about extending the barrel by silver soldering is still valid.]