Whats "aow", an why do you need a 200.00 dollar permit to put one on? I see guys with these on AR's an all kinds of stuff, I just bought one today to put on the AR...never heard any talk about a vertical foregrip being illegal....?
"Any Other Weapon" is a category of regulated firearms under the National Firearms Act of 1934 -- the same law that required federal tax and registration of Machine Guns, Short Barreled Rifles, Short Barreled Shotguns, and Suppressors (Silencers...if you just can't resist calling them that), and Destructive Devices (grenades and such). "Any Other Weapon" is a catch-all category for such things as pen-guns, cane-guns, brief-case-guns (any gun designed not to look like a gun), SMOOTH-BORE pistols, and pistols with a forward vertical grip.
Your vertical grip is completely legal on an AR-15 or any other rifle.
If you buy a gun that is sold as a pistol (like the pistol version of a GSG, or the pistol versions of AKs or ARs) and put a forward-mounted vertical grip on it you've created an unregisterd AOW.
By the way, 99% of the entire reason that the "pistol" versions of GSGs, AR-15s, and AK-47s, etc., etc. were developed was because folks wanted to own a version that had shorter barrels than the legal 16" limit -- often as the original military version did -- and the only way to get that without registering it as a Short Barreled Rifle, was to build it on a receiver registered as a pistol and to never NEVER attach a butt-stock.
I have a strong feeling that the reason the railed version of the fore stock for the GSG rifle won't fit easily on the pistol version is to try and keep folks from easily making this mistake. Many, many, MANY folks don't know these laws very well and just perhaps, when Joe-New-GSG-Pistol-Owner buys one of the railed forarms, maybe he will call someone to find out why it doesn't fit his gun.
-Sam