Home / Parts / Barrels / LMT® MRP™ Barrels / Monobloc® 13.9″ 5.56 NATO, Chrome-Lined
// HM Defense Monobloc® · 5.56 NATO · LMT® MRP™ compatible barrel
Monobloc® 13.9″ 5.56 NATO, Chrome-Lined
13.9″ HM Defense Monobloc® barrel in 5.56 NATO, converted for LMT® MRP™ upper receivers. One-piece 4150 CMV with an integral gas block and mid-length gas, chrome-lined bore.
In stock · ships shooter ready
Price on request
- SKU BBL-MB-B139CL-MRP
- Caliber 5.56 NATO
- Barrel length 13.9″
- Gas Mid-length
- Muzzle Thread protector
Ordered through the SOT Custom Shop — tell us your MRP™ upper and rail length and we’ll confirm fit before it ships.
// Specifications
The numbers
| Specification | Value |
|---|---|
| SOT SKU | BBL-MB-B139CL-MRP |
| Barrel maker | HM Defense |
| Caliber | 5.56 NATO |
| Barrel length | 13.9″ |
| Material | 4150 CMV (chrome-moly vanadium) |
| Profile | Stepped, one-piece with integral gas block |
| Twist rate | 1:8 |
| Bore | Chrome-lined |
| Finish | Black phosphate |
| Gas system | Mid-length |
| Gas block | Integral — machined into the barrel; no set screws or pins |
| Gas journal | None — integral block |
| Barrel extension / feed ramps | M4-style feed ramps |
| Barrel interface | LMT® MRP™ upper receivers |
| Muzzle threads | 1/2×28 |
| Thread protector | SOT Standard Barrel Thread Protector, fitted — CNC-machined steel, black or nitride finish |
| Muzzle device | None — fitted and timed to order |
| Bolt face | Standard 5.56 (.378) |
| Magazines | Standard 5.56 NATO magazines |
| Inspection | Headspace gauged; bore and crown inspected; recorded on SOT-BIR-001 |
| Availability | In stock |
| Price | On request |
// About this barrel
Built, converted, gauged.
The HM Defense Monobloc® is machined from one piece of 4150 chrome-moly vanadium: barrel, gas journal and gas block together. There is no gas block to index, no set screws to back out, no taper pins to shear and no clamp interface to shift under heat. The gas port is drilled to a known size in a fixed location and it stays there.
Mid-length gas at 13.9″ — lower port pressure and a longer dwell than carbine gas, which shows up as a softer impulse and less wear through the bolt carrier group. Every length in this family runs mid-length, so the rifle keeps the same character when you change barrels. Bore is chrome-lined; finish is black phosphate.
Converted to the LMT® MRP™ barrel interface, then inspected, gauged and assembled by the Second of Ten build team to a documented standard. Installs with the MRP™ upper’s factory barrel hardware — no proprietary tooling and no permanent modification to the receiver. Headspace is gauged and recorded on the barrel inspection record before shipment.
Ships shooter ready: gas tube installed and the muzzle protected by the SOT Standard Barrel Thread Protector (CNC-machined steel, black or nitride finish), with no muzzle device fitted. Brakes, flash hiders and suppressor mounts are fitted and timed to order through the SOT Custom Shop. A headspaced bolt carrier group can be added and gauged to this barrel: SOT Enhanced HM Defense in Black Nitride, or the SOT Enhanced Microbest Super-Stake with the Young Manufacturing HMB™ bolt.
// Includes
What ships
- 13.9″ Monobloc® barrel, converted to the LMT® MRP™ interface
- Mid-length gas tube, black nitride finish, installed
- SOT Standard Barrel Thread Protector, fitted (CNC-machined steel, black or nitride finish)
- Vacuum-sealed, with a Zerust® corrosion-inhibitor tube in the bore (remove before firing)
- Barrel inspection record (SOT-BIR-001)
// Options
Add to the build
- Headspaced bolt group: SOT Enhanced HM Defense, Black Nitride
- Headspaced bolt group: SOT Enhanced Microbest Super-Stake with Young Manufacturing HMB™
- Muzzle devices from other manufacturers, installed and timed — for a specific muzzle device, please ask before ordering
Every headspaced bolt group is gauged against this barrel and recorded with it.
// Matching headspaced bolt
Add a bolt, headspaced to this barrel
Any of these 5.56 NATO / 6mm Max® bolt face bolts can be added to this barrel. We gauge headspace with the bolt in this barrel and record the pairing on the barrel inspection record, so the two ship as a matched set.
| Bolt | Details | Price |
|---|---|---|
| Microbest C158 MPI bolt kit, chrome (phosphate extractor) | Bolt, extractor, firing pin, retaining pin and cam pin | $79.00 |
| HM Defense HMB™ bolt, black nitride | Cam pin socket in place of a through-hole | $135.00 |
| Young Manufacturing 5.56 M-16 bolt, black nitride | Item 1-50-12-301 | $89.95 |
| Young Manufacturing 5.56 M-16 bolt, chrome | Item 1-50-12-303 | $129.95 |
| Young Manufacturing 5.56 HMB™ bolt with cam pin, black nitride | Item 1-50-12-302 | $99.95 |
| Young Manufacturing 5.56 HMB™ bolt with cam pin, chrome | Item 1-50-12-300 | $149.95 |
Want a complete bolt carrier group instead? See the headspaced bolt groups under Options above, or ask the Custom Shop.
// Before you order
Fits LMT® MRP™ upper receivers only. Cut for LMT® MRP™-pattern upper receivers only — it will not fit a barrel-nut upper. Rail length governs clearance; tell us your receiver and rail and we will confirm fit before it ships.
Barrel under 16″. On a rifle-configured firearm it makes a short-barreled rifle. Since October 9, 2026, ATF no longer enforces NFA approval or registration for short-barreled rifles. That is an enforcement policy following the Jensen and Silencer Shop Foundation v. ATF rulings, not a change to the statute, and it could change. Taking an SBR across state lines still needs ATF approval on Form 5320.20, and state and local laws on short-barreled rifles still apply. The firearm’s configuration, and compliance with your state’s law, is the purchaser’s responsibility.
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.
Shipping. We do not ship barrel assemblies to California, Connecticut, Delaware, the District of Columbia, Hawaii, Illinois, Massachusetts, New Jersey, New York, Rhode Island or Washington, or to Denver, Colorado, or Cook County/Chicago, Illinois. Monobloc® barrels are HBAR-marked and can ship to Maryland. If you have questions, consult your local FFL.
Warranty. We warrant the conversion and the fit. A converted barrel is modified from its original configuration, so any coverage from the original manufacturer is at their discretion. If something goes wrong, come to us first.
// The rest of the range
Other LMT® MRP™ barrels
Monobloc® is a registered trademark of HM Defense. HMB™ is a mark of HM Defense, LLC, used under license. 6mm Max® is a registered trademark of BC Precision Ballistics. 6mm ARC is a trademark of Hornady Manufacturing Company. LMT® and MRP™ are marks of Lewis Machine & Tool Company. RIFLESPEED, SOLGW, Ballistic Advantage and Mos-Tek are marks of their owners. Second of Ten LLC is not affiliated with, sponsored by or endorsed by any company named here.