// Parts · Barrels
Barrels
The barrels we build our rifles and uppers on, in a wide range of calibers and lengths from 8″ to 24″ — from a roster of nine qualified makers, each chosen for a specific engineering role, and every barrel held to one acceptance standard at our bench.
// LMT® MRP™
LMT® MRP™ compatible barrels
Thirteen barrels converted for LMT® MRP™ upper receivers — Monobloc® 5.56 in four lengths, a 10.3″ government-contract clone, 6mm Max® and 6mm ARC. All in stock, all shooter ready, every one gauged and recorded before it ships.
- 13 in stock
- 5.56 NATO
- 6mm Max®
- 6mm ARC
- 10.3″ to 18″
Already have the barrel you want? Send it in and we will convert it to fit your MRP™ upper — $135, about two to three weeks.
// How we choose
We qualify makers, not barrels.
The barrel is the single component that most determines whether a rifle hits. Every other engineering decision we make — the billet receivers, the handguard, the bolt, the trigger and gas system — exists to let a barrel do its work without interference. So who makes the barrel is not a purchasing decision here. It is an engineering decision.
No single barrel maker is best at everything. Rather than force every rifle onto one vendor and compromise the build to fit, we qualify a short roster of makers — nine of them — each earning its place against a specific, documented engineering role.
Role before brand
Each build starts from a requirement — thermal stability, documented match accuracy, the broadest caliber availability, guaranteed value, a specific proprietary cartridge, or the smallest group at distance — and the roster supplies the maker who owns it.
Process over marketing
A maker earns its place on verifiable manufacturing: rifling method and twist uniformity, steel grade and stress relief, in-house operations, lapping, headspace and inspection discipline, and a track record we can put in front of an agency end-user.
One standard at the bench
Whoever cut the barrel, it is the same Second of Ten rifle once it is installed. Every barrel passes the same acceptance protocol and is installed to the same thermal-interference standard.
// Acceptance protocol
Every barrel, every maker, one standard.
Roster status earns a maker consideration. It does not exempt a single barrel from inspection. Every barrel that enters the shop is qualified individually, and recorded on our barrel inspection record (SOT-BIR-001), before it is fitted to a receiver.
- Inbound inspection and material verification. Each barrel is inspected on receipt against its specification — caliber, length, profile, twist, gas length, thread and finish — and its material confirmed against the build sheet.
- Bore and chamber verification. Bore condition, crown, and chamber and leade are verified — the surfaces that turn a good blank into a bad-shooting barrel if they are wrong, whoever made it.
- Dimensional and headspace confirmation. Critical dimensions and gas-port location are checked, and headspace is confirmed on the assembled bolt-and-barrel pairing — never assumed from the maker’s gauging alone.
- Thermal-interference fit. The barrel is installed to the Second of Ten thermal-interference standard, so the assembled upper is one rigid, dimensionally true structure, not a stack of toleranced parts.
- Function and accuracy validation. The completed upper is function-tested and validated against the Bombproof by Design™ protocol before it ships. The acceptance record travels with the build.
// The roster
Nine qualified barrel makers
Listed in the order they anchor our line. 6mm MAX is sourced from three of them — Sons of Liberty Gun Works, Shaw, and BC Precision Ballistics (built by Mos-Tek) for our compact 10.5″ builds — for redundancy and configuration choice on a specialty cartridge. In our complete firearms and complete upper receivers, 6mm Max® is offered with SOLGW or Shaw barrels, plus the 10.5″ BC Precision Ballistics barrel (built by Mos-Tek); 6mm ARC is offered with SOLGW, Ballistic Advantage, Shaw, Satern or Faxon barrels.
// 01
HM Defense
The integral-gas-block Monobloc.
The engineering centerpiece of the Second of Ten line. HM Defense mills the gas block directly into the barrel blank — the patented Monobloc — so the barrel is one homogeneous piece of steel with no clamped-on, dissimilar-metal interface. It heats and expands more evenly, holds its group longer as it works, and removes one serviceable interface that could loosen over a service life. Match-grade, triple-honed, button-rifled 4150 Chromoly Vanadium, headspace-gauged at the extension, mid-length gas at every length including 12.5″. When the build cannot compromise on thermal stability, the Monobloc is the answer.
Where we use it: SOTM4 RECC·E · SOTM4 MB · QUATTRO 15MB. Offered in our complete uppers and complete firearms, not as a bare barrel — with one exception: our LMT® MRP™ Monobloc® barrels, sold on their own or installed in a complete LMT® MRP™ upper receiver.
// 02
Criterion Barrels, Inc.
Match-grade button rifling, National-Record heritage.
Germantown, Wisconsin. Criterion began in 1999 as the button-rifling operation founded under Krieger’s roof, and is now fully independent. Its custom-built CNC button-rifling process delivers twist uniformity that rivals single-point cut rifling, and its barrels have set numerous National Records across Service Rifle, High Power, and CMP disciplines. Ordnance-grade 4150 chrome-moly and 410/416R stainless arrive double stress-relieved; every barrel is hand-lapped after rifling for a mirror bore that fouls less and cleans easily. Match-grade accuracy and barrel life at a value the discipline rarely sees — our reference for documented precision.
Where we use it: SOT MR-SASS · SPR / DMR-grade marksman builds
// 03
Faxon Firearms
Vertically integrated breadth — and the muzzle-device ecosystem.
Greater Cincinnati, Ohio. Family-owned, with a 45-year machining heritage — precision components since 1978 across automotive, aerospace, oil & gas, and defense — before launching Faxon Firearms in 2012. Barrels are made 100% in-house on Faxon’s own equipment: Duty Series in 4150 CMV with salt-bath nitride inside and out and M4 feed ramps; Match Series in 416R stainless with 5R rifling and an 11° crown. Every Duty barrel is stress-relieved, air-gauge tested, magnetic-particle inspected, and individually headspace-checked. Faxon also makes the suppressor-ready MuzzLok® muzzle devices we ship standard — one vertically integrated source for the broadest range of qualified barrels plus the muzzle ecosystem.
Where we use it: SOTM4 MB muzzle system · duty barrels across the lineup · 6mm ARC option
// 04
Ballistic Advantage
Duty-grade value with a sub-MOA guarantee.
Built within the Aero Precision manufacturing group, Ballistic Advantage pairs 4150 CMV (Modern and Performance Series, QPQ nitride) with 416R stainless (Premium and Premium Black, HP- and MPI-tested) and the proprietary Hanson profile that idealizes the length-and-gas pairing to shed weight without losing rigidity. Premium-tier barrels carry a sub-MOA accuracy guarantee and ship with Nickel-Boron-coated extended M4 feed ramps. Strong, consistent, widely available, and accuracy-guaranteed at a real-world price — our proven value tier when the program needs duty-grade barrels at scale.
Where we use it: Value-tier duty builds · multi-caliber options (incl. 6mm ARC)
// 05
Mos-Tek
Specialty & proprietary-cartridge barrels — the maker behind the 6mm Max®.
Mos-Tek is the precision barrel shop that builds the BC Precision Ballistics 6mm Max® barrel under contract — and a specialty-caliber maker in its own right. Owner-run and made in the USA, Mos-Tek designs, chambers, and profiles its barrels in-house in 416R stainless (Nitride, Melonite, or Cerakote), and is known for the work other shops turn away: custom contours, custom port locations, and proprietary, wildcat, and straight-wall chamberings. The 6mm Max® is the headline — a cartridge that exceeds 6mm ARC terminal performance through a barrel-and-magazine swap while keeping the Mil-Spec BCG and the unaltered .378 bolt face, for roughly 1,700+ ft-lb of muzzle energy with no modified bolt to source. When a build needs a specialty or proprietary chambering, Mos-Tek is the source that machines it.
Where we use it: SOT MK18 in 6mm Max · specialty straight-wall & wildcat builds
// 06
Satern Barrels
Benchrest-grade precision, proprietary in-house tooling.
Estherville, Iowa. Founded in 1997 by Steven and Debra Satern — Steve a competitive shooter and machine builder — Satern moved into barrel manufacturing in 2003 on machinery it designs and builds in-house. The result is a button- and cut-rifled, hand-lapped barrel with a reputation for accuracy well beyond 400 yards, trusted by competitive shooters, military, law-enforcement, and hunters alike. Available in stainless and chrome-moly, and one of our 6mm ARC barrel options. When the purpose of a build is the smallest possible group at distance, Satern’s hand-built, proprietary-tooled barrels are our long-range pick.
Where we use it: Precision / competition builds · extended-range marksman rifles · 6mm ARC
// 07
CarbonSix
The carbon-fiber lightweight tier.
CarbonSix builds custom carbon-fiber barrels on McGowen Precision button-rifled blanks — a match-grade stainless core turned to a reduced profile and wrapped in carbon fiber. The wrap is the point: it strips weight a steel profile can’t, adds rigidity, and dissipates heat so the barrel cools faster and holds its point of impact through long strings of fire. When a build’s deciding requirement is weight — front-end mobility for the recce and patrol role, or offsetting the muzzle weight a suppressor adds — CarbonSix keeps the rifle fast in the hands without giving up accuracy.
Where we use it: Lightweight recce / patrol builds · suppressor-weight offset
// 08
Sons of Liberty Gun Works
6 MAX & 6mm ARC barrels — the house that took the 6 MAX mainstream.
Sons of Liberty Gun Works didn’t design the 6 MAX — BC Precision Ballistics did — but SOLGW is the company that took it mainstream. SOLGW recognized the gap the 6 MAX fills (true 6mm terminal performance on an unmodified mil-spec .378 bolt face and full-mass BCG, built on a straight-walled 350 Legend parent case) and put real weight behind it: a published ballistic case for the cartridge, the Broadsword receiver sets and complete rifles built around it, and competition use in the 2024 gas-gun series. SOLGW’s PRECISION SPR barrels — 416R stainless, 3-groove polygonal rifling, SPR profile, mid- and rifle-length gas — are a primary Second of Ten supply for both 6 MAX and 6mm ARC. Its 16″ 6mm MAX barrel is offered in our SOTM4 MOD-0 complete upper receivers and in our complete SOTM4 billet rifles (billet upper and lower).
Where we use it: 16″ 6mm MAX SOTM4 MOD-0 complete uppers · complete SOTM4 billet rifles · 6 MAX & 6mm ARC SPR builds · marksman / GP-division gas guns
// 09
Shaw Barrels
The century-old OEM barrel house — 6mm ARC and 6 MAX barrels.
Shaw Barrels (formerly E.R. Shaw) is widely regarded as one of the largest barrel makers in the country — and one of the least visible, because the bulk of its output is OEM work cut for other brands. Founded over a century ago in Bridgeville, Pennsylvania, Shaw runs a high-volume operation across a vast caliber catalog, and catalogs a dedicated 6mm MAX barrel line in its own right. Second of Ten builds its 6mm ARC SOTM4 uppers on Shaw barrels — including the 18″ 6mm ARC SOTM4 complete upper receiver — and its 16″ 6mm MAX SOTM4 complete upper on Shaw’s 16″ 6mm Max® barrel (1:7.5″ twist, HBar contour, spiral fluted). The SOTM4 MOD-0 complete upper is offered with Shaw’s 18″ straight-fluted 6mm Max® barrel, and we also build on Shaw’s 18″ spiral-fluted 6mm Max® barrel (both 1:7.5″ twist, HBar contour, rifle-length gas); Shaw’s 6mm Max® barrels are also offered in our complete SOTM4 billet rifles and as an option in the SPIRE 6 series — a proven, deep-inventory source that widens configuration and availability choice on the platform.
Where we use it: 18″ 6mm ARC SOTM4 complete upper · 16″ 6mm MAX SOTM4 complete upper (spiral fluted) · SOTM4 MOD-0 complete upper (18″ straight-fluted 6 MAX) · 18″ spiral-fluted 6 MAX · complete SOTM4 billet rifles · SPIRE 6 series (6 MAX) option. Shaw 16″ 6mm Max® barrel (1:7.5″ twist, HBar contour, spiral fluted), converted for the LMT® MRP™, is coming soon.
// Sourcing matrix
Who we reach for, and why
| Maker | Role on the roster | Where we use it | Material and process |
|---|---|---|---|
| HM Defense | Flagship duty-precision hero barrel — single-piece gas system, maximum thermal stability. | SOTM4 RECC·E · SOTM4 MB · QUATTRO 15MB | 4150 CMV · button-rifled · triple-honed · headspace-gauged · mid-length gas (all lengths) · Black Cerakote® |
| Criterion Barrels, Inc. | Documented match accuracy and barrel life over high round counts. | SOT MR-SASS · SPR / DMR-grade marksman builds | 4150 ordnance CMV / 410 · 416R stainless · double stress-relieved · button-rifled · hand-lapped · headspaced |
| Faxon Firearms | Widest qualified catalog of lengths / calibers / profiles, plus integrated suppressor-mount muzzle devices. | SOTM4 MB muzzle system · duty barrels across the lineup · 6mm ARC option | 4150 CMV (Duty, QPQ nitride in & out) / 416R 5R (Match) · stress-relieved · air-gauged · MPI · headspace-checked |
| Ballistic Advantage | Proven duty / precision value tier — sub-MOA guaranteed, high availability. | Value-tier duty builds · multi-caliber options (incl. 6mm ARC) | 4150 CMV (Modern/Performance, QPQ) / 416R stainless (Premium, HP + MPI tested) · Nickel-Boron M4 feed ramp |
| Mos-Tek | Specialty / proprietary-caliber and one-off barrels — including the contract-built BC Precision 6mm Max®. | SOT MK18 in 6mm Max · specialty straight-wall & wildcat builds | 416R stainless (Nitride / Melonite / Cerakote) · custom contours, ports & twists · made in USA |
| Satern Barrels | Long-range / competition precision tier — smallest achievable group at distance. | Precision / competition builds · extended-range marksman rifles · 6mm ARC | Button- and cut-rifled · hand-lapped · stainless / chrome-moly · proprietary in-house machinery |
| CarbonSix | Ultra-lightweight carbon-fiber tier — maximum weight reduction and heat dissipation without losing accuracy. | Lightweight recce / patrol builds · suppressor-weight offset | Carbon-fiber-wrapped match-grade stainless core (McGowen blank) · button-rifled · reduced contour · custom lengths |
| Sons of Liberty Gun Works | 6mm ARC & 6 MAX SPR barrels — the maker that brought the 6 MAX to the broader market. | 16″ 6mm MAX SOTM4 MOD-0 complete uppers · complete SOTM4 billet rifles · 6 MAX & 6mm ARC SPR builds · marksman / GP-division gas guns | 416R stainless · SPR profile · 3-groove polygonal rifling · mid / rifle gas · made in USA |
| Shaw Barrels | High-volume OEM barrel house — 6mm ARC and 6 MAX barrels, with a deep caliber catalog. | 18″ 6mm ARC SOTM4 complete upper · 16″ 6mm MAX SOTM4 complete upper (spiral fluted) · SOTM4 MOD-0 complete upper (18″ straight-fluted 6 MAX) · 18″ spiral-fluted 6 MAX · complete SOTM4 billet rifles · SPIRE 6 series (6 MAX) option | 6mm ARC and dedicated 6 MAX barrel lines · 416R stainless · HBar / precision contours · century-old high-volume OEM · made in USA |
// Beyond the roster
This roster is where we start — not where we stop.
Second of Ten works with most of the major barrel manufacturers, and other options are available based on your specific requirements. If your build calls for a maker, material, profile, or chambering not shown here, reach out and detail your need — we will source and qualify the right barrel for it to the same Second of Ten standard.
// SOT Custom Shop
LMT® MRP™ & MWS barrel conversions
Lewis Machine & Tool builds the MRP™ and MWS around a proprietary quick-change barrel system — which is exactly why the barrel an owner wants is often one the platform won’t accept off the shelf. We close that gap: send us the barrel you want to run — an AR-15 barrel for the MRP™, an AR-10 barrel for the large-frame MWS, any length and any caliber — and it comes back converted to fit. That includes barrels with no commercial path onto the platform, such as the gas-block-integral HM Defense Monobloc®.
MRP™ $135 and MWS $175 per barrel, about two to three weeks in the shop.
// Questions
Talk to us about barrels
Tell us what you are building and we will help you choose the right part.
Monobloc® and HMB™ are marks of HM Defense, LLC, used under license. 6mm Max® is a mark of its owner. LMT®, MRP™ and MWS® are marks of Lewis Machine & Tool Company; Second of Ten is not affiliated with or endorsed by LMT®. Other product names are marks of their respective owners.
Bombproof by Design™
Precision // Purpose // Performance