HM Defense's Monobloc HM-15

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It's not that often that anything really new emerges in the AR world. Sure, we see new innovations on an almost daily basis but rarely is it something that relates to the fundamental design of the firearm. HM Defense's Monobloc HM-15 is truly something different, an AR15-style carbine with both a gas tube interface and muzzle device machined integrally into the barrel. If you believe that simpler is better, the HM-15 Monobloc is a serious step in your direction.

HM Defense emerged a few years ago out of an existing aerospace machining business in Mt. Orab, Ohio. Because HM Defense already had significant machining capabilities and expertise, they were able to build all of the major components from the ground-up rather than sourcing parts from the few forging and machining houses that feed much of the AR industry.

HM Defense solves the problem of NFA compliance by machining an integral flash hider into the barrel. By doing so, the barrel meets the 16" requirement for rifles without adding any excess length and requires no pinning or welding to achieve.


When you're used to building complex components for clients such as GE's aircraft division, maintaining the tolerances necessary for producing firearms is pretty straightforward. As HM began producing gun parts, they applied their engineering talent toward looking for ways to improve them; the Monobloc was one of those innovations.


Because HM produces their own button-rifled barrels, the barrel is one of the major components that they looked at with a critical eye. They saw the traditional gas block as not only a failure point, an unnecessary part that could potentially come loose, they saw the task of installing gas blocks as a barrier to more streamlined production.

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Traditional gas blocks use staking, set screws, or pinning to keep the unit in place; all of these methods usually work but if they fail, the firearm becomes a very slow and nearly useless pull-bolt action. Suppressors are an all the more common accessory on AR-style rifles and suppression can put increased stress on the gas block.

To avoid these issues, HM deleted the gas block from their design altogether, removing the failure point and allowing for simple and rapid gas tube installation during construction. The result of their engineering is increased simplicity and simple is almost always better when it comes to reliability and durability.


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There are two gas tube options available for HM's Monobloc. One is their quick change gas system, only available on their complete rifles, which uses a removable screw to retain the gas tube. In the event that the gas tube needs to be removed or replaced, the operation can be performed in seconds with a simple hex tool.

The second option, available on component barrels and complete uppers, uses a traditional roll pin to attach the forward end of the gas tube to the barrel. The Monobloc is the lowest profile gas system that I have even seen on an AR, with the highest point of the block protruding just .280" above the barrel.


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The low-profile system allows for a slim and trim forend, our test rifle was fitted with an M-LOK capable free float tube aluminum produced by HM. This forend is smooth and snag-free except for the top, which has an integral full-length Picatinny rail.

The Monoblocs are available in various configurations and lengths, but our test rifle was in the style that I prefer-- a mid-length gas system coupled with a 14.5" barrel. The National Firearms Act dictates that a rifle must have a barrel of at least 16" in order to avoid registration. To achieve the legal 16 inches with a 14.5" barrel, a muzzle device of 1.5" or longer must be permanently attached.

An aluminum heat sink helps draw heat away from the steel barrel and also serves as an attachment point for the handguard.

"Permanent attachment" usually means that a flash suppressor or brake must be pinned and welded onto the barrel. HM Defense avoided that step by machining their own flash hider into the barrel itself. The only drawback to this design is for folks who want to attach suppressors to proprietary flash hiders such as those produced by AAC or Surefire. For those customers, HM offers traditional threaded Monobloc barrels.

To test the accuracy and reliability of the HM-15 Monobloc, we mounted a Vortex HD Gen II 4.5-27x56mm optic. I wouldn't mount such a massive scope to a lightweight carbine for daily use, but to ensure that we were shooting the HM-15 to its full accuracy potential, this high quality precision rifle optic was the way to go.

The Monobloc's gas tube interfaces directly into a port milled into the barrel itself, eliminating the need for a traditional gas block. Not only does this make gas tube installation incredibly simple, it means that the gas block will never be a failure point.

Accuracy was excellent with two out of the three loads tested, both Hornady's 55gr. VMAX load and Israeli Military Industries' 77gr. OTM achieving repeatable sub-MOA 5-shot groups. Our testing took place on a day that saw triple-digit temperatures and we didn't give the barrel much time to cool in between strings of fire. This didn't seem to matter, as some of the best groups of the day were shot with a scorching-hot bore.

The barrel itself uses a 1-7" rifling twist so it can stabilize just about any weight .224" bullet on the market. In our testing, this combination of twist rate and barrel length maintained its accuracy across a broad spectrum of bullet weights which is always good news.

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The barrel nut does triple duty on this carbine, not only securing the barrel to the receiver but acting as an attachment point for the free-floating rail as well as a heat sink. A heat sink is a dissimilar metal (in this case, aluminum) used to pull heat away from a critical part. The aluminum heat sink draws heat away from the steel barrel and allows the air to cool over a series of fins which are used to increase the surface area of the part.

HM isn't the only AR manufacturer to incorporate a heat sink (JP Enterprises uses them as well, among others), but it is a sure sign of a premium build.

Rifles with carbine-length gas systems may look high-speed, but the increased heat and pressure of the shortened gas system can be a detriment to reliability and longevity. Unless a rifle is a true SBR and requires a carbine gas system, I prefer a mid-length setup.

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Thanks to the mid-length gas system, and a gas port that was obviously correctly-sized, the carbine was very easy to shoot from a recoil perspective. ARs that are over-gassed can cause the bolt carrier to slam rearward with unnecessary force, which makes for slow follow-up shots. On the other hand, insufficient gas pressure produces guns that are unreliable- this carbine struck the right balance.

We did not experience any malfunctions at any point during our testing. Our test model was fitted with a Velocity trigger, which is considered a dealer-installed option. The single stage trigger broke consistently at 3.5 pounds with no discernable creep.

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One thing to note is that the HM-15's billet aluminum lower was built to be compatible with Gen 2 PMAGs so Gen 3 versions are a very tight fit due to the close tolerances. USGI aluminum magazines work fine as well as Lancers, Hexmags, and others. HM also offers a traditional forged lower, which works with all AR magazines.

The HM-15 is not just another box-built black rifle. Virtually the only major parts of the HM-15 that aren't produced in house by HM Defense are the MOE stock and pistol grip from Magpul. The lower receivers, barrels, bolt carrier groups, and even various small parts (including a QD end plate that HM Sells as a component part) are produced under one roof.

The Monobloc uses a low profile free float aluminum handguard with a 12 o'clock rail and M-LOK compatible vents.

That's actually pretty unique in the AR marketplace, where most builders assemble rifles from outsourced parts. There's nothing wrong with that and, honestly, it would be cost-prohibitive to build guns from scratch if a company didn't have other sources of revenue paying for the expensive CNC machines.

As much as this review is about a carbine, it's also about the innovation of a small American business. When smart people apply their resources toward making better products, we the consumers get to reap the rewards of those innovations. The companies take the risks in hopes that the marketplace will make their designs profitable. HM Defense is just one of many companies producing quality products that give us (and hopefully our troops), the best firearms that our efforts can produce.

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