One of the best quadrails available - HERA Arms 110502 IRS AR15 Rifle Aluminum Handguard Black Hard Coat Anodized 9"

while I did have to file down one of the anti-rotation tabs in order to get this to fit with my Aero upper, this is still second only to the Cross Machine Tool quadrails in quality (and it's less than half the cost). The barrel nut provides a sturdy attachment for the handguard, and like all good barrel nuts does not require indexing. The biggest downside is that the upper ridge is too narrow to allow the us of a superlative arms piston system, although any standard DI gas block should be fine.