Home / Parts / Bolt Carrier Group Parts / SOT Enhanced Chrome Bolt
// Microbest · 5.56 · Bolt
SOT Enhanced Chrome Bolt
A bolt, a cam pin and a firing pin. One of them tells you it went back in the right way round.
Standard geometry
$89.00
- SKU BLT-MBEST-C158-CHR-E
- Bolt steel C158, shot peened
- Testing HPT & MPI
- Finish Chrome
- Bolt face Mil-spec .378
Questions about fit? Tell us the rifle and the barrel and we’ll confirm the right part before it ships.
// The operator’s case
The bolt is the part of an AR that is asked to do the most and given the least room to do it in.
It carries the entire chamber pressure on seven lugs, cams through a quarter turn under load, and does it again every time the rifle cycles. Which is why the two things that matter about a bolt are what it is made from and whether anybody actually tested it — not the finish, and certainly not the logo.
This is the Microbest bolt: C158 steel, shot peened, then High Pressure Tested and Magnetic Particle Inspected individually, in a chrome finish. Microbest builds bolt carrier groups for the military and for other small arms manufacturers, and this is the same bolt that goes into their Super-Stake carrier groups, offered on its own for a builder who already has a carrier.
Second of Ten supplies it Enhanced: with our arrow-index-marked cam pin and a firing pin, so what arrives is the assembly you actually install rather than the one part of it. The mark on the cam pin is the point. Everything else in the package is standard, correct and unremarkable, which is exactly what you want.
What the Enhanced version actually changes
Nothing mechanical. The bolt is the Microbest bolt, unmodified, carrying its manufacturer’s warranty in full. What changes is that one thing which was previously invisible becomes visible: which way the cam pin went in.
Note that the index-marked gas key we put on our complete bolt carrier groups does not apply here. A bare bolt has no gas key. That mark belongs to the carrier, and it appears on the SOT Enhanced Super-Stake Bolt Carrier Group.
We did not make the bolt better. We made one mistake harder to make.
// Why it
We made one mistake harder to make.
01 The 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. Our cam pin head carries the SOT arrowhead with SOT inside it, and 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 the moment the mistake gets made, and exactly the moment an unmarked pin tells you nothing. It costs nothing to look at.
02 C158, and tested
C158 is the mil-spec bolt steel, shot peened to put the surface into compression so cracks have to start against the grain of the residual stress. Then each one is High Pressure Tested and Magnetic Particle Inspected.
03 HPT and MPI are a pair
HPT proves the bolt survived an overpressure round. MPI then looks for the crack that proof round might have started. Either one alone tells you considerably less than people assume — a bolt that passed proof and was never inspected afterwards has an untested claim on it.
04 Chrome where it wears
Chrome is slick and gives carbon very little to hold onto, which matters most on the part that runs inside a carrier packed with fouling. It also makes the bolt easy to wipe clean without solvents and easy to inspect — you can see a problem on a bright surface.
05 A firing pin in the box
Supplied with the bolt rather than sold alongside it. A bolt without a firing pin is a part; a bolt with one is an assembly you can actually install. This is a packaging decision, not an engineering claim, and we would rather say so plainly.
06 Mil-spec .378 bolt face
The standard AR-15 face, which also runs 6mm MAX, .300 BLK and .350 Legend. It is not a 6mm ARC or 6.5 Grendel bolt — those case heads are larger and take a different part.
07 What this is not
This is a standard-geometry bolt: the cam pin passes through a bored hole, as it has on every AR bolt since the 1950s. If you want the geometry where that hole becomes a blind socket and material is retained at the cam pin location, that is the HMB™ bolt, licensed from HM Defense, and it is a different part on a different sheet. Both are good bolts. They answer different questions, and we would rather you bought the right one.
08 Gauged on the rifle it goes in
Where we fit this bolt to a build, headspace is gauged and checked against that barrel rather than assumed from supplier paperwork. A bolt and a barrel are a pair, and they are only right together.
// Specifications
The specification
| Specification | Value |
|---|---|
| Product | SOT Enhanced Chrome Bolt — Microbest C158 MPI chrome bolt, with SOT arrow-index-marked cam pin and firing pin |
| SOT SKU | BLT-MBEST-C158-CHR-E |
| Bolt steel | C158, shot peened |
| Testing | High Pressure Tested (HPT) and Magnetic Particle Inspected (MPI), individually |
| Finish | Chrome |
| Bolt face | Mil-spec .378 — also runs 6mm MAX, .300 BLK and .350 Legend |
| Cam pin location | Standard pass-through hole. For the socket geometry see the HMB™ bolt |
| Supplied with | SOT arrow-index-marked cam pin (arrow point faces toward the bolt face) and a firing pin |
| Extractor and springs | Fitted: extractor, extractor spring and ejector spring included |
| Carrier compatibility | Any mil-spec bolt carrier. No proprietary carrier required |
| Price | $89.00 |
// Before you order
Bolt face compatibility. This is a mil-spec .378 bolt face. It is not interchangeable with 6mm ARC or 6.5 Grendel, whose case heads are larger. 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, tell us the barrel and we will tell you the bolt.
Fitment and inspection.
- Degrease and lubricate before installation.
- 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.
- Install the cam pin with the arrow point facing toward the bolt face.
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. The SOT mark is applied to the cam pin head and does not alter any load-bearing dimension. Second of Ten will act as the owner’s agent in any claim against that coverage where it runs broader than ours, and warrants its own fitting and gauging work. 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. Microbest, Sprinco 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 are as published by the manufacturer for the bolt carrier group containing this bolt and were verified on 18 AUG 2026; they 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