The AR-15 is America’s longest serving service rifle and has a deserved reputation for performance and reliability. However, as with any machine, parts can fail. While not a particular weak point (10,000 round counts are a reasonable minimum) AR-15 bolts can break. One potential point of failure is the center hole fitting the cam pin.

HM Defense designs, develops, and produces AR rifles and parts that feature a number of their own patent-pending technologies. The company manufactures and final machines rifle components in-house in their Mt. Orab, Ohio facility. To improve bolt longevity, HM Defense designed their HMB Bolt, an enhanced durability part for M16/AR-15 and AR-10 rifles. The HMB Bolt corrects a natural weak point in existing bolts where the cam pin hole passes through. Under extensive use, standard bolts can fail there. The HMB Bolt eliminates the pass-through cam pin hole and replaces it with a socket and tapered pin. This reinforces a natural weak point by significantly increasing the amount of metal and strength at the cam pin location.

HMB Bolts are drop-in replacements and made of 9310 steel with a 4340 hardened steel cam pin and Nitride black finish. Additionally, the HMB Bolt reduces contact between cam pin and firing pin, enabling smoother operation of firing pin, bolt lugs and aft gas rings. The new cam pin socket also retains lubrication for extended operations. The AR-15 model works with 5.56 NATO, .223 Remington, and .300 AAC rifles. This bolt comes standard on all HM Defense HM15 rifles and is available as a bolt replacement kit or in a complete bolt carrier group.

“The HMB Bolt design provides a substantial improvement in long-term durability over the existing Mil-Spec bolt,” said Clay Barker, General Manager at HM Defense. “Every M16 and AR rifle can benefit from our new HMB TM Bolt design.”

For more information, contact HM Defense (937/444-6500, HMDefense.com).

Read more in the May 2018 issue.

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