I recently received a question about having a gunsmith work on a NFA firearm. Although the law states that only the owner may be in possession of a NFA firearm, there is a small exception to the rule for a licensed gunsmith or manufacture of firearms with a Class 3 SOT. While an ATF Form 4 is used to transfer a NFA firearm, it is not necessary to transfer the firearm to the gunsmith using a Form 4 because the gunsmith's or manufacture's FFL authorizes the gunsmith to be in possession of NFA firearm. HIstorically, individuals or trusts have used an ATF Form 5 for these types of transfers but their are not required and generally not done becaue of the long delay in optaining the approval.
In addition, not any gunsmith or manufacture is authorized to work on NFA firearms. In order to do so, a gunsmith must have a Class 3 Special Operations Tax (SOT) license. Further, a gunsmith with a 01 FFL is only authorized to perform, let's call them, "basic" or "cosmetic" repairs to a firearm. On the other hand, a 07 FFL is authorized as a manufacturer. A 07 FFL would be able to perform modifications to the firearm, such as cutting down the barrel, replacing the barrel, adding additional parts, and other major modifications.
Taking your NFA firearm to a FFL with a Class 3 SOT in the same state, or intrastate, does not require any additional paperwork, and is a relatively simple process. ATF recommends a 5320.20 but there is no requirement for one. However, sending a NFA firearm to a FFL with a Class 3 SOT in another state, or interstate, requires an ATF Form 5320.20 to be approved by BATF. If you are mailing the firearm through a common carrier, it is important that the carrier has a copy of the 5320.20 form as well. If you are using a common carrier in the same state, they will need a copy of the ATF 5320.20 to be in possession during transportation of the Title II firearm.
After the FFL has completed the repairs, make sure you go get your firearm! Newly passed legislation requires that if you leave any firearm with a FFL for more than 30 days, you must fill out a Form 4473 again. A form 4473 is the same form that must be filled out when you originally purchase a firearm from a FFL holder and includes state mandated background checks as well as the NICS check.