Here is a chance to benefit from my lack of research. I ordered one of these from Rainier Arms for a build I am doing on a Mega Billet upper receiver. Well it turns out that these will only fit on standard upper receivers becuase of a little tab at the rear of the handguard that I assume is for anti rotational purposes.
It has been removed from it package, and I screwed the barrel nut on my upper by hand, but did not torque it. I immediately realized that it would not fit, and removed it. It comes with the original box and accessories. You can read the specs here: http://www.rainierarms.com/?page=shop/detail&product_id=1710
It is just like new, and I paid $187.20 including shipping. I will sell for $165 shipped. E-mail is the best way to contact me. Please add 3% for PayPal. Thanks for looking!