Home / Parts / Barrels / LMT® MRP™ Barrels / Monobloc® 14.5″ 5.56 NATO, Chrome-Lined
// HM Defense Monobloc® · 5.56 NATO · LMT® MRP™ compatible barrel
Monobloc® 14.5″ 5.56 NATO, Chrome-Lined
14.5″ 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.
One in stock · ships shooter ready
Price on request
- SKU BBL-MB-B145CL-MRP
- Caliber 5.56 NATO
- Barrel length 14.5″
- Gas Mid-length
- Muzzle Flash hider pinned and welded
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-B145CL-MRP |
| Barrel maker | HM Defense |
| Caliber | 5.56 NATO |
| Barrel length | 14.5″ |
| Overall length | 16.2″ with the flash hider pinned and welded |
| Material | 4150 CMV (chrome-moly vanadium) |
| Profile | Stepped, one-piece with integral gas block |
| Twist rate | 1:8 |
| Bore | Chrome-lined |
| Finish | Black Cerakote® |
| 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, polished |
| Barrel interface | LMT® MRP™ upper receivers |
| Muzzle threads | 1/2×28 |
| Thread protector | None — a HUXWRX flash hider is pinned and welded instead |
| Muzzle device | HUXWRX Flash Hider-QD 556, 1/2×28, XL (13.7″), pinned and welded; HX-QD suppressor mount |
| 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 — 1 available |
| 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 14.5″ — 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 Cerakote.
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 a HUXWRX Flash Hider-QD 556 XL pinned and welded for a 16.2″ overall length, with the HX-QD suppressor mount — because the flash hider is pinned and welded, this barrel ships without the thread protector. 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
- 14.5″ Monobloc® barrel, converted to the LMT® MRP™ interface
- Mid-length gas tube, black nitride finish, installed
- HUXWRX Flash Hider-QD 556, 1/2×28, XL (13.7″), pinned and welded — overall length 16.2″
- 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™
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.
16.2″ with the flash hider. The HUXWRX flash hider on this 14.5″ barrel is pinned and welded — a permanent attachment that brings the overall barrel length to 16.2″, so it counts as a 16″-plus barrel — which still matters in states that restrict short-barreled rifles and when you travel across state lines. Removing the flash hider would leave a 14.5″ barrel, which on a rifle-configured firearm makes a short-barreled rifle. ATF no longer enforces NFA approval or registration for SBRs (an enforcement policy since October 9, 2026), but Form 5320.20 for interstate travel and state law still apply. The firearm’s overall configuration 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.