In MY opinion? No.
In the ATF's opinion? Yes.
The ATF has taken the view that a forward grip on a pistol makes it an AOW (any other weapon). I disagree
The wording of the statute actually says that a pistol is a gun that was "originally" designed or made to be fired with one hand. Unless the gun comes from the factory with that foward pistol grip, then I don't see how adding one changes how it was "originally" designed or made to be fired.
Let's use a Glock pistol as an example. Gaston Glock clearly originally designed or made them to be fired with one hand. Some of them also have a rail on the front. The gun doesn't ship with a forward grip. But if you buy an aftermarket one and install it, the ATF says it's an AOW.
But here's the kicker: because the definition of pistol says ORIGINALLY designed or made to be fired with one hand, how does your aftermarket addition change what was original to the gun? The original design and manufacture are unchangeable once the gun is in existence and on the market. No matter what you do with it, it's still originally designed or made to be fired with one hand.
Of course, you can't add a stock and say "Hey, it's still a pistol because of how it was originally designed or made." But that's because there's a definition of an SBR in the statute, and it's specific enough to encompass pistols with stocks, and is more restrictive. But the definition of AOW is not specific enough to automatically encompass pistols with forward grips. It is a catchall section.
So in my opinion, a pistol with an aftermarket forward grip is not an AOW. But in the ATF's opinion it is.
One of these days, I ought to register a pistol as an AOW, pay the tax, put on my forward grip, and then sue the ATF in the US Court of Claims to get my money back. That would give us all case law to point to when we want to add a forward grip.
Aaron