CMMG 30DC30A Barrel Sub-Assembly 300 Blackout 16.10" Stainless Bead Blasted Finish 416R Stainless Steel Material Carbine Length with Medium Taper Profile for AR-15
http://www.tandktactical.com/shop/815835016245-cmmg-30dc30a-barrel-sub-assembly-300-blackout-16-10-stainless-bead-blasted-finish-416r-stainless-steel-material-carbine-length-with-medium-taper-profile-for-ar-15-540
http://www.tandktactical.com/web/image/product.template/540/image_1920?unique=93a3b7a
CMMG's Barrel Sub-Assembly is ideal for building your custom AR-15 upper group. It comes with properly installed M4 barrel extension.
