The Achilles’ Heel Of The AR Bolt

The Achilles’ Heel Of The AR Bolt
A mil-spec AR-15 bolt broken at the cam pin hole.

A great option for beefing-up your mil-spec AR bolt.

Ever notice that the AR-15 bolt has a great big, honking hole drilled right through it? And that hole has sharp edges that have to be carefully de-angled or it’s not fun to work with? If that edge gets nicked, the bolt will die and soon. Why do we use it? Because the government cast that design in stone back in 1968. If you stick with mil-spec, you stick with the bolt. But, should you?

HM Defense AR Bolt vs MilSpec
HM Defense bolt (center) versus mil-spec (right).

How about we improve that? HM Defense did just that. They took the simple measure of not drilling the hole all the way through. I know, simple, right?

The HM Bolt simply has a blind hole for the cam pin. This does, of necessity, require a new cam pin—one short enough not to stop short in the hole—but that’s easy. How does this make a bolt stronger, and why do we care?

Simple: By drilling the bolt body through, the bolt makers (who have no other choice) drastically weaken the bolt body. With most of it gone, there are only two thin webs to support the gas rings, firing pin guidance and hold the bolt together. I’ve seen bolts cracked and broken at the cam pin hole. It isn’t a subnuclear detonation, but it does stop the rifle from working.

Broken AR Bolt
A mil-spec AR-15 bolt broken at the cam pin hole.

By stopping short of going through, HM Defense gives the thin side walls support from the far side of the bolt. Even if the stresses try to crack the bolt at the thin webs, the webs are supported by the far side being solid.

Also, regular bolts have to have the cam pin hole edges staked on the far side or else they could be assembled backward. I saw it—once—and the stream of invectives from the owner (he had fired the one shot he’d get on the rifle stage of a 3-gun match) was impressive.

The bolts are made to be a drop-in fit or replacement to all mil-spec bolt/carrier setups and given the proper heat treatment, testing and finish to provide lots of use. Each one comes with an appropriately shortened cam pin, so you won’t have to try and grind one down on your bench grinder to make it fit.

Does this matter? Sure. We all try to make our equipment as durable and reliable as we can. We test things, we check regularly and some worry (more than others) over what might go wrong.

HM Defense HMB Bolt

Anything we can do to reduce the number of things we have to worry about is a good thing.

Plus, the price of the HM Bolt is right in line with regular AR-15 bolts (MSRP is $175). Yes, parts are hard to come by right now, but when they get a bit easier, you might want to upgrade the parts list on your next build.

Editor's Note: This article originally appeared in the June 2021 issue of Gun Digest the Magazine.

More On Upgrading The AR-15


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  1. Mr. Hofstra, thank you, and to the rest of you gun aces out there, I’d like your opinions, too.
    I did not speculate about inserting a long bolt that would fill the hole. I speculated about installing a SHORT bolt at the open bottom.
    As to Loctite, many have had bad results, but only because they didn’t use the proper red kind.
    I cannot agree that the hole would be widened with my idea. The threads you cut are filled back in by the bolt threads.

  2. Interesting article. My comment though is on the whole series of related articles, which I have bookmarked and shared to friends that are looking to go to the AR platform.
    As I’ve commented in the past, 10+ years ago, I purchased my first AR, and I admit I was totally unfamiliar with the platform, and I went cheap with a DPMS Oracle; the epitome of budget AR’s. Long story short, it took a considerable amount of money to turn it into a reliable gun. The only parts that are original now, are both the upper and lower receivers. Everything else has been replaced/upgraded. The moral of the story is that it’s possible to go too cheap, and I caution any Newbie as I was, to research and join the forums before you buy. While it was a good learning experience, it was an expense I could have avoided had I known more or researched more.

    I appreciate the articles gentleman and the commentary, it’s from you all that I’ve learned and I thank you all for that.

  3. Oops, I made a typo: I didn’t want to write “blind hold”, I wanted to write “blind hole”, as the author did. (A blind hole is a hole that does have a solid bottom, not air at its bottom end.)

  4. Hmmm … could you:
    a) Cut threads in the other end of the hole, then
    b) thread in a very short, flushheaded allen head bolt into the other end of the hole?
    If so, the open end would be converted into the blind hold featured in this product, the owner’s old rifle bolt would be made stronger, and the head of the bolt wouldn’t stick out. I don’t know how much a gunsmith would charge … if it’s not a bad idea … but many of us are already pretty good at cutting threads and wouldn’t have to spend any money at all,except for the allen head bolt.

    • Doing this wouldn’t have the effect of strengthening the bolt – the hole is still there – it’s just filled with a allen head bolt. In fact the hole is made even larger by threading it and the allen head bolt would likely back out or thread further in, even with Loctite applied – stopping function of the weapon. Just another thing to cause a breakdown.


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