If you do it yourself and not register it.. your looking at 10 years in prison.
To me... it is not really worth turning a pistol into a AOW for so much money. Unless your looking to have something your friends do not have.
It appears that only the state of MA does not allow AOWs.
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.