FN Manufacturing 16 Inch Government Profile ( A2 ) Hammer Forged barrel. Barrel has 5.56 Nato chamber,  1/7 twist rate, M4 barrel extension, carbine length gas system, threaded ½ x 28 muzzle and Mil-Spec phosphate coated.  Barrel and chamber are chrome lined to a specification that is almost double the thickness of a standard M4/M16 Mil-Spec barrel and are stamped with the FN oval .  These barrels are hammer forged, machined, chambered, chrome lined and completely phosphated by FN Manufacturing.

18 Inch Mid-Length Fluted Stainless Steel Barrel with Extension and Pin 
.223 Wylde Chamber, air gauged, cryo-treated 1:8 twist, threaded 1/2-28, with barrel extension and pin. Weight 37.3 ounces.

Rock River Arms 16 inch Air-Gauged, cryo-treated Stainless Steel, Fluted, H-Bar barrel chambered in .223 Wylde with a 1:8 twist, .750 gas journal diameter, mid-length gas system, and a target crown muzzle.

The .223 Wylde chamber was designed as a match chambering for semi-automatic rifles. It will accomodate both .223 Rem and 5.56mm NATO ammunition. It is relieved in the case body to aid in extraction and features a shorter throat for improved accuracy.

5.56mm Lo-Pro Gas Block Barrel with Switchblock®

Cold Hammer Forged, Chrome Lined Barrel -5.56mm

--1 in 7" twist 
--Extended feed ramps 
--Manganese Phosphate finish 
--Pinned Switchblock
--1/2x28 threads 
--Light Contour

 To allow access to the Switchblock®, it cannot be covered up by a handguard. The longest handguard that can be used with a carbine gas system is 7" and a 9" for the mid-length, intermediate, and rifle length.