Next comes removal of the rifle's gas system, which consists of a gas block and gas tube. Our gas block was the clamp-on style. Loosen the two tension screws and you can pull the block/tube assembly off the barrel together. If you have a Mil-Spec A2 front sight post, there are two large pins to drive out.
Next comes removal of the rifle’s gas system, which consists of a gas block and gas tube. Our gas block was the clamp-on style. Loosen the two tension screws and you can pull the block/tube assembly off the barrel together. If you have a Mil-Spec A2 front sight post, there are two large pins to drive out.

Green Mountain originally focused on muzzleloader barrels, but now offers barrels for nearly everything that goes “bang”—including things that don’t, such as air gun

barrels. A major supplier of barrels to countless gun makers, as well as the public directly, Green Mountain makes some of the best AR barrels you can buy. Case in point: the 16-inch, medium profile, fluted, .300 BLK barrel ($167.95) we chose for our own conversion project. If you want one for your own project, you can order one online at www.gmriflebarrel.com.

Whether you have a Mil-Spec or aftermarket barrel nut will dictate the type of wrench you'll need to remove and reinstall the nut. Once the nut has been fully unscrewed, simply pull the barrel free of the upper receiver
Whether you have a Mil-Spec or aftermarket barrel nut will dictate the type of wrench you’ll need to remove and reinstall the nut. Once the nut has been fully unscrewed, simply pull the barrel free of the upper receiver

The .300 AAC Blackoutcartridge (7.62x35mm) was developed by Advanced Armament Corporation in cooperation with Remington Defense to achieve ballistic performance in the M4 carbine similar to that of the 7.62x39mm Russian cartridge while using standard STANAG magazines at their full capacity. Basically a slight modification of the.300 Whisper developed in the early 1990s by J. D. Jones of SSK Industries, the .300 AAC Blackout’s SAAMI short name is “.300 BLK”.

At this point, we're ready to introduce our new .300 BLK barrel to the rifle's upper. Before mating, Daemon Defense recommends applying a light coating of grease (Shell 33MS Aircraft grease, to be exact) to the barrel extension to prevent galling and to ease future disassembly, should the need ever arise.
At this point, we’re ready to introduce our new .300 BLK barrel to the rifle’s upper. Before mating, Daemon Defense recommends applying a light coating of grease (Shell 33MS Aircraft grease, to be exact) to the barrel extension to prevent galling and to ease future disassembly, should the need ever arise.
Our new Green Mountain .300 BLK barrel uses the shorter carbine-length gas system (bottom). Since the .556 barrel we removed operated with a "mid-length" gas system (top), our barrel swap necessitated the use of a carbine-length gas tube.
Our new Green Mountain .300 BLK barrel uses the shorter carbine-length gas system (bottom). Since the .556 barrel we removed operated with a “mid-length” gas system (top), our barrel swap necessitated the use of a carbine-length gas tube.

The primary development goal for the .300 BLK was to create a reliable, compact, low-recoiling, .30-cal solution for the AR platform using both sub-sonic ammunition for optimal sound and flash suppressed fire and supersonic ammunition that matched the ballistic performance of the 7.62x39mm cartridge. Other goals included the use of the existing inventory of 5.56 NATO magazines while retaining their full 5.56 NATO capacities, and to provide urban war fighters with the ability to penetrate barriers with high-mass projectiles.

The popularity of the .300 BLK among hunters and shooters has increased geometrically as more gun and ammunition manufacturers have jumped on the bandwagon. It’s hard to find an AR maker that does not offer a .300 BLK in some form (complete rifle, upper half or both), and most ammo brands offer .300 BLK, usually in both sub-sonic and super-sonic versions. Sub-sonic ammunition appliesheavier bullets at lower muzzle velocities, primarily used in suppressed weapons. Super-sonic ammo employs lighter bullets at super-sonic muzzle velocities, which are best suited to hunting.

Page 3