The ATF is very clear on adding a vertical grip to your pistol. ($200 if you do it yourself, $5 if a NFA manufacturer does it.) either way it is a AOW requiring taxation. So here is the question, What if you add a horizontal grip?
To lawfully add a vertical fore grip to a handgun, a person must make an appropriate application on ATF Form 1, “Application to Make and Register a Firearm.” The applicant must submit the completed form, along with a fingerprint card bearing the applicant’s fingerprints; a photograph; and $200.00. The application will be reviewed by the NFA Branch. If the applicant is not prohibited from possessing a firearm under Federal, State, or local law, and possession of an “AOW” is not prohibited in the applicant’s State of residence, the form will be approved. Only then may the person add a vertical fore grip to the designated handgun.
A person may also send the handgun to a person licensed to manufacture NFA weapons. The manufacturer will install the fore grip on the firearm and register the firearm on an ATF Form 2. The manufacturer can then transfer the firearm back to the individual on an ATF Form 4, which results in a $5.00 transfer tax. If the manufacturer is out of State, the NFA Branch will need a clarification letter submitted with the ATF Form 4 so that the NFA Branch Examiner will know the circumstances of the transfer. Questions can be directed to the NFA Branch or the Firearms Technology Branch.
Seems like splitting hairs to me, Yes I do agree that it does say Vertical but after all the legality HubBub you would have and the lawyers argue over the wording, just would not be worth the hassle. If you just have to have one for the cool factor and are a patient person, find a Class2 that you can send it too and let them register it and transfer it back on a Form4, but if you ever wanted to get rid of it would have to go thru the long wait of AOW paperwork to sell it to somebody.