The HM Defense MonoBloc Barrel:

We’ve all heard the “Just as good!” notions from internet gunsmiths. Casuals will regularly circulate the myth that building a quality AR is as simple as pushing two pins to mate an upper and lower receiver. Candidly, entrusting the technical installation procedures of a rifle’s critical components to a budget-friendly third party may result in a malfunctioning rifle at best, or a rifle that fails catastrophically at worst.

One all-too-common failure point of the internal piston AR-15 stems from the gas block. The gas block directs propellant gasses from the fired cartridge to cycle the action. Naturally, if the gas block is improperly installed, it may restrict or redirect the gasses away from the action, thus causing malfunctions. Therefore, it is crucial to install a gas block with due diligence and thorough oversight. 

A contemporary low profile gas block will be injection molded or cast out of some steel correlate. Secured by two small set screws, the gas block’s ability to seal and direct gasses to the system is reliant upon its manufacturing tolerances (how tight it hugs the barrel) and how well the set screws can grip the round surface of the barrel. This may seem counterintuitive; how are screws supposed to dig into a round, convex surface? 

Sadly, your bullshit meter is not misfiring. Run-of-the-mill ARs will have oversized gas blocks poorly fitted onto barrels where they will loosen, shift, and restrict gas pressure needed to cycle the action. Some manufacturers have elected to “dimple” the barrel to create concave points where the fasteners can dig into respective holes, thus reducing shift. This process works, albeit still reliant upon the tolerances and materials of the gas block, the torque applied to the set screws, and whether thread locker was applied to keep said screws from loosening under the heat and vibration of the firing sequence.

Dimpled barrel for gas block set screw.

By far the most secure of gas block installation procedures is drilling and pinning in place. The block is aligned and torqued with set screws, then the barrel and gas block are drilled, reamed, and an oversized cross pin is pressed into place, thus preventing the gas block from shifting under load. This process is still reliant upon the materials used and the tolerances of the block. If the block was easy to slide onto the barrel, then the high velocity propellant gasses will partially escape from the block, thus reducing positive pressure into the system. If the gas block and the barrel were made of different steels, then the respective thermal expansion properties may differ, thus causing undue stress on the system and potentially increasing gas loss during the firing cycle. This process is also time and labor intensive, and rarely comes as an OEM offering.

Drilling and pinning a gas block is secure and effective, but still creates some headaches.

Enter the HM Defense MonoBloc barrel. This patented unit features an integrated gas block machined from the same chunk of steel used to make the barrel. The method is fascinating: First, HM turns down a solid carbon steel blank to shape the exterior profile. The barrel is subsequently placed into a six-axis CNC and the gas block is formed through a series of computer-directed drilling and machining processes. This procedure creates the lowest profile gas block in its class; no mounting hardware is needed as the gas block is the barrel. You obtain perfect gas block alignment, zero thermal expansion shift, zero gas loss, and none of the installation headaches. Most importantly, you receive peace of mind. Nothing is going to prevent your AR from losing gas port pressure when you need it most.

The three stages of HM barrel production: Blank, turned, and machined.

Our pièce de resistance is exclusive to Lead & Steel. We receive these barrels “in the white” where they are sent to our world-class plating facility to be melonited. This process impregnates nitrogen into the steel and considerably improves fatigue strength (resistance to dynamic loading), wear resistance, and corrosion resistance. The added surface hardness triples the expected barrel life, leaving you with a service life upwards of 40,000rds. Talk about longevity.

To conclude: The modern gas block is dead. The MonoBloc is here to stay. Pre-orders are live, and more barrel lengths and calibers are being developed every day. Consider us for your next build.

As always, stay sharp.


2 thoughts on “The HM Defense MonoBloc Barrel:”

  1. Hi, I recently bought the mono block barrel but it didn’t come with a gas tube. What size gas tube should I buy and do you sell the set screws to secure the gas tube. Thanks

    • Hey Omar!

      The MonoBloc barrel doesn’t come with a gas tube, we have mid length gas tubes listed separately that come with roll pins. If you ordered one with your MonoBloc barrel and it did not arrive, please reach out to customer service and we will get your sorted!



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