If you are considering adding a vertical fore grip to a handgun, then you should consult GRL Law first.
A “handgun” under federal law is designed to be held and fired with one hand.
If you add a fore grip to it, then it is no longer designed to be held and fired by using one hand.
It would now be classified as “any other weapon” (AOW) under the National Firearms Act. That weapon must be registered with the ATF.
Making an unregistered AOW is punishable by a fine and 10 years’ imprisonment. So is possession of unregistered AOW.
You can register to make an AOW by submitting ATF Form 1. The fore grip can only be lawfully added after the application is approved.
If you have questions about this or other firearm issues, then call GRL Law for more information.