The firearm must be registered because such a firearm constitutes “any other weapon” (AOW) as defined in the National Firearms Act (NFA) at 26 U.S.C. § 5845(e).

To lawfully add a vertical fore grip to a handgun, a person must submit an ATF Form 1, Application to Make and Register a Firearm along with a fingerprint card bearing the applicant’s fingerprints, a photograph, and $200 making tax. An applicant’s ATF Form 1 will be approved if the applicant is not prohibited from possessing a firearm under federal, state, or local law, and the applicant’s possession of AOW is not prohibited in the applicant’s state of residence. An applicant may only add a vertical fore grip to the handgun designated on his/her Form 1 only after the applicant’s Form 1 is approved by ATF.

[26 U.S.C. § 5845(e)]