Home / Parts / Bolt Carrier Group Parts / HMB™ Bolt — 5.56 / 6mm MAX Chrome
// HM Defense · 5.56 / 6mm MAX · Bolt
HMB™ Bolt — 5.56 / 6mm MAX, Hard Chrome
The hole was always the weak point. So it is not a hole any more.
$159.00
- SKU BLT-HMB-556-CHR
- Chambering 5.56 · 6mm MAX
- Bolt face Mil-spec .378
- Finish Hard chrome
- Cam pin Socket, radiused pin
Questions about fit? Tell us the rifle and the barrel and we’ll confirm the right part before it ships.
// The operator’s case
Every standard AR bolt is drilled straight through at the point it is most likely to break.
The cam pin hole passes clean through the bolt body, and it does so exactly where the bolt is asked to carry its highest load. Generations of shooters have accepted that because it is simply how the rifle was drawn in the 1950s. The HMB™ answers it with geometry rather than material science: the pass-through hole becomes a socket, and the cam pin that sits in it is radiused rather than square-shouldered.
The result is that material is retained at the cam pin location where a through-hole removes it, the contact between cam pin and firing pin is reduced, and the socket holds lubricant in a place a through-hole cannot. It is one change, in one place, aimed at the single most documented failure point on the platform — which is the whole of the Bombproof by Design™ argument in miniature.
The work that is not on the parts list
Bought as a bare bolt, this is a component. Fitted by us, it is part of a system: headspace gauged against the barrel it will run behind, and the bolt inspected before it goes anywhere near a carrier.
Every bolt ships with the SOT arrow-index-marked cam pin, bare or fitted, so the pin cannot go back the wrong way round. Where we supply the HMB™ inside a complete bolt carrier group it also carries the second of our additions — an index line across the gas key and both screw heads, so a screw that begins to back out is visible before it becomes a stoppage.
The bolt is HM Defense’s engineering. What happens to it before it ships is ours.
// Why it
The hole was always the weak point.
01 A socket, not a hole
This is the entire product in one sentence. Where a mil-spec bolt is bored through, the HMB™ carries a blind socket, so the steel that a drill would have taken out stays in the bolt at precisely the location that sees the most stress. Note the wording carefully: the design retains material rather than adding it. Nothing is bulked up, nothing is oversized, and the bolt is compatible with any mil-spec bolt carrier, dropping in exactly as the part it replaces does.
02 A radiused cam pin
The socket runs with a radiused cam pin rather than a square-shouldered one. A radius spreads load into the surrounding steel instead of concentrating it at a corner, which is where cracks prefer to start. The two parts are designed together; neither is a drop-in improvement on its own.
03 Less cam pin to firing pin contact
The geometry reduces contact between the cam pin and the firing pin. Two hardened parts rubbing against each other under cycling loads is wear you get nothing for — removing it costs nothing and returns service life.
04 The socket holds lubricant
A through-hole lets oil pass straight out the other side. A socket keeps it where it was put. That is a small thing on a clean range day and a real one several hundred rounds into a dusty one.
05 Hard chrome finish
Hard chrome is applied after heat treatment, so the finish sits on a fully hardened bolt. It is the harder, slicker surface of the two and the easier one to wipe carbon off — the choice for high round counts and suppressed use.
06 Licensed, not imitated
The HMB™ is HM Defense’s design, built under license. This is not a workshop copy of a patented geometry — it is the actual part, and the license is why we can name it.
07 Know your bolt face before you order
This bolt is 5.56 mm NATO with a Mil-spec .378 bolt face. Also runs 6mm MAX, .300 BLK and .350 Legend. The 5.56 bolt and the 6mm ARC bolt are different parts and will not substitute for one another — the ARC case head is derived from 6.5 Grendel and is larger. If you are unsure which your barrel is chambered for, ask us before ordering rather than after. We would rather answer an email than process a return.
08 Gauged on the rifle it goes in
Where we fit this bolt to a build, headspace is gauged and checked against that barrel — not assumed from supplier paperwork. A bolt and a barrel are a pair, and they are only right together.
09 Every bolt ships with the SOT arrow-index-marked cam pin
A cam pin can be reinstalled the wrong way round, and a rifle will run for a while before it tells you. Every HMB™ bolt we sell — bare or fitted — includes our arrow-index-marked cam pin, its head carrying the SOT arrowhead with SOT inside it. The arrow point faces toward the bolt face. That is a reference you can check with the bolt out of the rifle and out of the carrier, which is exactly when the mistake gets made. It costs nothing to look at and removes an error that is otherwise invisible until something goes wrong.
// Specifications
The specification
| Specification | Value |
|---|---|
| Product | HMB™ Bolt — 5.56 / 6mm MAX, Hard chrome |
| Manufacturer part number | 1-50-12-300 (Young Manufacturing) |
| Made by | Young Manufacturing, under the HMB™ design licensed from HM Defense, LLC |
| SOT SKU | BLT-HMB-556-CHR |
| Chambering | 5.56 mm NATO, 6mm MAX, .300 BLK and .350 Legend — all share the mil-spec .378 bolt face |
| Bolt face | Mil-spec .378. Also runs 6mm MAX, .300 BLK and .350 Legend. |
| Cam pin location | Socket in place of a pass-through hole; material retained at the cam pin location |
| Cam pin | Radiused |
| Carrier compatibility | Any mil-spec bolt carrier. No proprietary carrier required |
| Finish | Hard chrome |
| License | HMB™ design licensed from HM Defense, LLC |
| Included | SOT arrow-index-marked cam pin — arrow point faces toward the bolt face |
| Supplied as | Standalone bolt, or fitted inside a complete SOT bolt carrier group |
| Price | $159.00 |
// Before you order
Bolt face compatibility. The 5.56 and 6mm ARC HMB™ bolts are not interchangeable. The 6mm ARC case head is derived from 6.5 Grendel and is larger than the mil-spec .378 face used by 5.56 mm NATO, 6mm MAX, .300 BLK and .350 Legend. Fitting the wrong bolt is a headspace and safety problem, not an inconvenience. If there is any doubt about what your barrel is chambered for, contact us before ordering.
Fitment and inspection.
- Where SOT fits the bolt, headspace is gauged and checked against the barrel it will run behind.
- Where the bolt is supplied bare, headspacing and inspection are the purchaser’s responsibility and should be done by a qualified armourer before firing.
- The HMB™ is compatible with any mil-spec bolt carrier. It does not require a proprietary carrier, and it does not change the rifle’s external dimensions or controls.
WARNING: This product can expose you to chemicals including lead, which is known to the State of California to cause cancer and birth defects or other reproductive harm. For more information go to www.P65Warnings.ca.gov.
Warranty. The bolt is supplied unmodified and carries its manufacturer’s warranty in full. Second of Ten will act as the owner’s agent in any claim against that coverage where it runs broader than ours. Where SOT has fitted and gauged the bolt, that work is covered under the SOT Terms of Sale. See warranty information.
// Related
Related parts and groups
All bolt carrier group parts and the comparison → Bolt Carrier Groups →
Bombproof by Design™ and Gundaddy® are marks of Second of Ten LLC. HMB™ is a mark of HM Defense, LLC, used under license. Young Manufacturing, 6mm ARC and all other trademarks are the property of their respective owners and are used for identification of components only, without implying endorsement or affiliation. Specifications and pricing are subject to change without notice. Any bolt should be headspaced against the barrel it will be used with by a qualified armourer before firing.
Bombproof by Design™
Precision // Purpose // Performance