The legal definition of a handgun under federal law is...
The term "handgun" means- (A) a firearm which has a short stock and is designed to be held and fired by the use of a single hand; and. (B) any combination of parts from which a firearm described in subparagraph (A) can be assembled.
The key part is designed to be fired with one hand... which means when you add a forward grip to a handgun it is no longer a handgun. It becomes a AOW because it is concealable on ones person and is not a handgun. Just like a pen gun, wallet gun, cane gun etc.