For an example of why gun magazines are not authoritative, I refer everyone to page 58 of Guns, August 2008 issue. There you will see a Kel-Tec PLR-16 with a Surefire vertical foregrip attached. Here's my letter to the editor:
Interesting article on the Kel-Tec PLR-16. Even more interesting photo on page 58 with the Surefire light and vertical foregrip attached. Iím sure youíre aware that attaching a vertical foregrip to a pistol makes it a prohibited NFA weapon, and is illegal under federal law if the appropriate tax stamp is not obtained.
Hereís a link for you: http://www.atf.treas.gov/firearms/041006-vert_grip.htm. Not sure if you registered the weapon in the photos as an AOW, but you might consider warning your readers before they follow the example in the photo and unwittingly make themselves into felons eligible for 20 years in federal prison.
We had a good chuckle about it around the office, but the possibility exists that someone unfamilar with the law will add a foregrip to his "assault pistol" and find himself in deep kimchi.