Rev. Rul. 55-570

A gunsmith desires to affix a sleeve-type muzzle brake to the muzzle end 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 by 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 the Internal Revenue Code of 1954