Forgive me for tagging on an old thread, but if someone is doing research into this issue and finds this thread in a search, I want to be sure that they have clear and accurate information.
Attaching a forward pistol grip to any handgun makes that handgun an "Any Other Weapon" strictly regulated by the ATF. To legally attach the forward pistol grip you would have to file paperwork and pay a $200 tax. There have even been cases of people being prosecuted - and convicted - for merely having a pistol grip which could be attached to the front of a handgun, even though they never actually mated the two.
ATF got in trouble recently for prosecuting such cases, but that doesn't mean they won't still try to do it, especially if you don't have some other gun the fore-grip can be legally attached to.
It's all just crazy and ATF is notoriously capricious about enforcement.