HM Defense AR-15 Rifles: A Superior Design?

There are a lot of AR-15 manufacturers out there making rifles with custom touches intending to improve the gun in some way. This could be a souped-up trigger, custom handguard design, rail configuration, or many others. HM Defense focused their design changes on the gas system and bolt on the AR-15. My first experience with an HM Defense rifle was the MC5 Raider 556, which remains one of my favorite guns of all time.

This gun isn’t just a high-quality rifle, it has some modifications that make it more reliable over time. These modifications would be the HM Defense Monobloc barrel and HMB bolt. I spoke about these two patented parts in my review of the MC5 Raider a few years ago. I’ve carried the MC5 at work, trained with it, and had lots of fun on the range. But are these custom changes needed on an AR-15 rifle?

To be honest, I had forgotten about the patented bolt and barrel on my gun. I would remember when I stripped it down to clean it but quickly forgot again. But over the past year, I’ve seen some AR-15s fail from broken bolts and gas tube issues. This may not be a common problem with AR-15 rifles, but it makes you wonder if or when it could happen to yours. Let’s break the MC5 down for a close look at these two parts.

HMB Patented Bolt

HM Defense removed the passthrough hole for the cam pin from the bolt. On a standard bolt, the cam pinhole travels through the entire bolt (see photo below), creating a weak point for potential cracks over time. During my first armorers’ course, inspecting the bolt for cracks was one of the first things we were taught to do during inspections.

Again, this may not be a huge issue with most rifles, but it does happen from time to time. By leaving the other side of the bolt enclosed, it eliminates some of the weak points on the bolt. Most of the cracked bolts I’ve seen came from guns that were still functioning. However, I’ve observed one bolt inside the BCG fall apart and jamb during a training exercise. A chunk fell out right on the side of the cam pinhole.

I’m not an engineer by any means, but I don’t see how the HMB bolt could have the same issue. The cam pin is slightly shorter on the HMB to accommodate the closed bolt side. This also helps reduce contact between the firing pin and cam pin during operation. Other than that, everything else on the HMB bolt uses mil-spec parts.

Monobloc Barrel

For me, this is the most interesting change HM Defense has made to their AR-15 rifles. Instead of a traditional gas block that mounts to the barrel, the gas block is part of the barrel. They did this by taking a solid barrel blank and milling them out of the same piece of metal. Instead of screwing the gas block on from the bottom, the gas block is integrated with the barrel and cannot move.

One of the tricky parts of installing the gas block is getting it aligned properly. If it’s off even a small amount, it will keep the gun from cycling properly. Once aligned correctly, you also must take some type of measure to keep the screws from coming loose. I have always used Blue Loctite to keep them on, but the HM defense method eliminates that need completely.

Besides the monobloc technology, HM Defense also uses high-grade materials to make its barrels. The 4150 Chromoly Vanadium barrel has a 1:8 twist ratio and a chrome-lined bore. A manganese phosphate-coated exterior gives it a good, durable finish. Every HM Defense barrel is match-grade, triple-honed, and button-rifled.

The only downside I’ve seen with this system is that you need to remove the gas tube from the gas block any time you need to remove the barrel. Because the gas tube attaches to the gas block, you can normally slide it off as one piece from the barrel. Unless you plan on disassembling your gun regularly, however, this won’t be an issue.

Other Features of HM Defense Rifles

HM Defense uses forged 7075-T6 lower receivers with a Type III Class II hard-coat, black anodized finish. The upper receiver is made from 7075-T6 M4 aluminum with the same anodized finish as the lower. My Raider MC5 has M4 feed ramps and a standard 1913 rail on top. During my time on the range with the MC5, I’ve used a variety of AR-15 magazines, including Magpul, Lancer, KCI, Hex Mag, and more, without issue.

You can purchase just the barrel from HM Defense if you want to take advantage of the monobloc technology. Retail on the barrels starts at $349.99, with barrel lengths ranging from 12.5 to 16 inches. Besides their AR-15 rifles, HM Defense also produces AR-10s and even a 50BMG (HM50B) for those wanting a little bigger bullet. Retail on the HM50B is $5,499.99

Is it a superior design?

From what I see of their products, HM Defense makes a great-quality gun with some interesting upgrades. Are these upgrades enough to call them a superior design? That is a complicated and hard question to answer because there are so many good manufacturers out there. I think it’s safe to say they are making some innovative changes, and time will tell if they truly outperform other brands.

What sets them apart without argument is their customer service. I’ve said time and time again that a company is only as good as its customer service, and HM Defense is a leader in this field. Located in Ohio, they make their product in the USA, and if you call, they pick up the phone. It won’t be a computer, but a real person who will ask you how your day is going. If there is an issue with a product, they will make it right.

During some reviews, I like to call the company I’m reviewing just to see if they answer the phone. When I called HM Defense, someone answered on the second ring and was able to answer all my questions. I also ran into a group from HM Defense at a restaurant during SHOT and had the privilege of speaking to them in person. They are a passionate, family-oriented group that focuses on making the best product they can for people like us.