Frequently, clients will inquire of whether he/she can change the barrel length of a short-barreled rifle or shotgun (SBR or SBS), after receiving the Bureau of Alcohol, Tobacco, Firearms and Explosives’ (ATF) approval of his/her Form 1 or Form 4 on which a specific barrel length listed.
18 U.S.C. § 921(a)(8) provides, “The term “short-barreled rifle” means a rifle having one or more barrels less than sixteen inches in length and any weapon made from a rifle (whether by alteration, modification, or otherwise) if such weapon, as modified, has an overall length of less than twenty-six inches” and Section 921(a)(6) provides, “The term “short-barreled shotgun” means a shotgun having one or more barrels less than eighteen inches in length and any weapon made from a shotgun (whether by alteration, modification or otherwise) if such a weapon as modified has an overall length of less than twenty-six inches.” Similarly, the National Firearms Act, in essence, defines it the same way in 26 U.S.C. § 5845(a), although it is broken down slightly different.
Accordingly, the law concerns itself with barrel length being under 16″ for a rifle and under 18″ for a shotgun or an overall length under 26″ for either a rifle or shotgun. Once registered as a SBR or SBS, the actual length is somewhat immaterial, as it has already been registered, as required by law, as an SBR or SBS. Furthermore, the ATF has previously issued a determination that an individual may turn an SBR or SBS back into a normal Title I gun, without need for notifying the ATF. Hence, after registering a SBR or SBS, you may change the barrel length to some other length still constituting an SBR/SBS, such as changing it from a 14.5″ barrel to an 11.5″ barrel, without violating the law and you may lawfully change the barrel length making it no longer an NFA firearm, such as changing it from a 14.5″ barrel to a 20″ barrel, without violating the law.
The one exception to the above is where the caliber is also being changed, whereby the firearm would now constitute a destructive device. Section 5845(f)(2) defines one type of a destructive device as “any type of weapon by whatever name known which will, or which may be readily converted to, expel a projectile by the action of an explosive or other propellant, the barrel or barrels of which have a bore of more than one-half inch in diameter, except a shotgun or shotgun shell which the Secretary finds is generally recognized as particularly suitable for sporting purposes.” Accordingly, if you were modifying a 50 Caliber SBR into a 20mm SBR, you would be in violation of the law to make such a firearm, if you did not receive the approval of ATF, registering the intended firearm as a destructive device (Destructive Devices trump Short-Barreled Rifles/Shotguns).
Hopefully this will clarify the issue of whether you may change the barrel length of your SBR/SBS.