mortablunt said:
The only exception is if the AR has an under 16" barrel and no stock. That (I might be wrong so correct me if I am) would turn it into a short barreled rifle, which requires registration.
A firearm that comes from the manufacturer as a RIFLE (not a handgun or "Other Firearm) cannot be fitted with a barrel less than 16" in length, and also cannot be configured in any way to be less than 26" overall with stock extended in the firing position. Doing either creates a Short Barreled Rifle which must (first) be registered under Title II of the National Firearms Act of 1934.
Under federal law (and most states' laws) adding a vertical foregrip to a rifle is perfectly legal.
An AR-15 sold without a shoulder stock and with a length less than 26" (regardless of barrel length) is a
handgun under federal law, and legal in most states. You cannot install a vertical foregrip on a
handgun or it becomes a Title II "Any Other Weapon," which requires an NFA Title II tax stamp just like a short barreled rifle. (See Post #4.)
According to current BATFE policy, you can change a handgun into a rifle, and back again -- so long as you don't add a barrel shorter than 16" AND a shoulder stock at the same time -- but you cannot turn a rifle into a handgun. So what configuration the gun was sold in originally actually matters quite a bit.
Until/unless BATFE interpretation changes again, you could have a VFG installed on your "rifle" that was built on a pistol, but NOT installed on the same gun when it is in pistol configuration.