For those of you new to the firearms world, the federal government (in their infinite wisdom) decided that any rifle with a barrel less than 16" was a 'Short Barreld Rifle' or SBR, and that it was very scary and required a bunch of paperwork (= money in the form of a federal tax). The standard M4, as issued to the military, has a 14.5" barrel and as such there's a lot of 14.5" barrels floating around out there. But, unless you pay your $200 fee and register your firearm as an SBR (or you're building an AR pistol), you cannot legally put that barrel on your AR, or build an AR with that barrel.
The solution is to permanently affix a muzzle device to the end of the barrel, bringing the effective length of the barrel to over 16". The ATF specifies that this is conducted by drilling a hole through the side of the muzzle device and into the threads in the barrel, then inserting a pin into that hole, and welding over the top of it. As crazy as it may sound, this is very commonly done, and we can usually get a 'pin and weld' turned very quickly and headed back to you.
But first, a few clarifications:
- Barrel length, as defined at the federal level, is the distance from the bolt face when the bolt is fully in battery, to the end of the muzzle. This is checked by fully closing the bolt, then dropping a rod cut to exactly 16" down the barrel. If ANY part of that rod extends out the muzzle, the barrel is less than 16" and it is illegal (felony) on an unregistered rifle.
- Please be aware that this process is a pretty intensive operation for your relatively delicate muzzle threads. You should absolutely assume that this is a permanent modification to your firearm. When we remove a pinned device, we will machine out the pin as carefully as possible, but it's always possible that the underlying threads have been damaged, or weld media may have gotten down in the threads. If that's the case, there's really nothing we can do, and as we unscrew the muzzle device it could destroy the threads. At that point, the barrel is already too short to cut off the threads, rethread, and still use a standard length muzzle device to bring it back to 16" overall.
- Along the same lines as the previous caution, muzzle device technology is among the fastest changing sector in the firearm world. We would recommend thinking long and hard about whether saving 1.5" off the overall length of your firearm is worth locking yourself into a single muzzle device, basically forever.