I spoke with a regional ATF Agent regarding the designation of bare receivers when purchased from distributors as such. The following is the information I received.
Regarding an AR Pistol and my current build of a Bolt Action Specialty Pistol and their legal classification, I did some homework.
I called:
ATF Field Agent E. Mata
Houston, TX ATF Field Division
https://www.atf.gov/houston-field-division(281)716-8200
regarding their classification and subsequent need for reclassification. The info is as follows.
Regarding stripped AR/Bolt Action/Break Action Receivers. If the bare receiver is made and sold from the factory as a bare receiver and classified as "Receiver" on the 4473 and "Other" on the NCIC Check from the FFL holder...........It (the receiver) does NOT need to be reclassified or any paperwork submitted by the end user once the receiver is manufactured into either a rifle or pistol by the attachment of a corresponding stock/grip or appropriate length barrel.
It is classified as a receiver and stays that way as long as all rules and regulations regarding barrel length or overall length are adhered to. For instance......if placed in a rifle stock, it must have a 16" barrel and so forth.
The only time the receiver will/must be reclassified as either a pistol or rifle is if it is later sold by an FFL dealer and an additional 4473 is completed. At that time, it will be reclassified in accordance with its current condition. Either a rifle or a pistol.
Clarification being what did it leave the factory as?
If it left the factory as a bare receiver, the end user "manufactures" it into either a rifle or pistol. If it leaves the factory already "manufactured" as a rifle or pistol............it is what it is and the end user cannot change that.........without money and paperwork.
Once it is "manufactured" by either the company or end user from its bare "receiver" status into one or the other (rifle/pistol)........any subsequent 4473s will reflect its end state.
Speculation can lead you to trouble. If you have questions...........call those that will be grinding your ass if you break the law.
