$149, Troy MRF Rail Black AR-15 7" SRAI-MRF-C7BT-00

The Troy MRF is the strongest user-installed free-floating rail system in the world. Troy MRFs are available for all M16/M4 weapon systems equipped with the original factory barrel nut. The MRF's patented mounting system reinforces the barrel nut with a sturdy clamp and anti-rotation alignment lugs. Troy MRF BattleRails provide an uninterrupted rail extension and hand guard aligned perfectly with the host weapon's flat-top receiver.- 7" MRF-C Rail System for all M4/M16 Type Carbine.- Adds only 4 oz. to weight of M4.- Accepts Quick Detach 360 push-button swing swivel.- BlackFinish/Color: BlackFit: AR-15Model: MRFSize: 7"Type: Rail

Source: http://www.manventureoutpost.com/products/Troy-MRF-Rail-Black-AR%252d15-7%22-SRAI%252dMRF%252dC7BT%252d00.html?google=1