This is what 2A Armament says:
 
 
A simple weight saving component for our AR15 BL rails . The aluminum BL rail barrel nut is made from 7075-T6 Aluminum, and then anodized black. This nut will save an additional .65oz over the standard titanium barrel nut.

    SKU (2A-ALBN-1)
    Weight 1.10 oz