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LMT® MRP™ Compatible Barrels
Thirteen barrels converted for LMT® Monolithic Rail Platform (MRP™) upper receivers, across five barrel families and three chamberings. Every one ships shooter ready — gas tube installed, and our thread protector fitted (two one-of-a-kind barrels ship with a muzzle device instead) — and is gauged and recorded before it leaves.
- All thirteen in stock
- Monobloc® 5.56
- Govt clone 5.56
- 6mm Max®
- 6mm ARC
In stock, not in a queue
Every configuration on this page is on the shelf. A cataloged barrel ships without waiting for a conversion.
Converted by a specialist
The MRP™ interface is machined by a specialist LMT® conversion shop. It installs with the upper’s factory hardware and indexes like a factory LMT® barrel.
Shooter ready, gauged, written down
Gas tube installed, thread protector or muzzle device fitted. Headspace gauged, bore and crown inspected, gas block pinned where one is fitted, and the barrel recorded on our inspection record.
// 6 in stock
Monobloc® 5.56 NATO
Six configurations, four lengths, mid-length gas at every length. Barrel, journal and gas block machined from one piece of 4150 CMV — nothing to clamp, pin or come loose. Sold on their own, or installed in a complete LMT® MRP™ upper receiver — ask us for an upper quote.
HM Defense
Monobloc® 12.5″ 5.56 NATO
12.5″ 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.
- 5.56 NATO
- Mid-length
- Integral block
- 1/2×28
Price on requestBBL-MB-B125-MRP
HM Defense
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.
- 5.56 NATO
- Mid-length
- Integral block
- 1/2×28
Price on requestBBL-MB-B139CL-MRP
HM Defense
Monobloc® 14.5″ 5.56 NATO
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.
- 5.56 NATO
- Mid-length
- Integral block
- 1/2×28
Price on requestBBL-MB-B145-MRP
1 in stock
HM Defense
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.
- 5.56 NATO
- Mid-length
- Integral block
- 1/2×28
Price on requestBBL-MB-B145CL-MRP
1 in stock
HM Defense
Monobloc® 16″ 5.56 NATO
16″ 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.
- 5.56 NATO
- Mid-length
- Integral block
- 1/2×28
Price on requestBBL-MB-B160-MRP
HM Defense
Monobloc® 16″ 5.56 NATO, Chrome-Lined
16″ 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.
- 5.56 NATO
- Mid-length
- Integral block
- 1/2×28
Price on requestBBL-MB-B160CL-MRP
// 1 in stock
Government-contract clone 5.56 NATO
A 10.3″ barrel built to the drawing for the US Army barrel, in commercial production. CRANE-spec gas port, runs on a standard carbine buffer.
Government-contract clone
Government-Contract Clone 10.3″ 5.56 NATO
10.3″ 5.56 NATO government-contract clone, converted for LMT® MRP™ upper receivers. SOCOM HEAVY profile, chrome-lined 41V50, CRANE-spec gas port. Designed to pair with the LMT® 9.25″ MRP™ upper.
- 5.56 NATO
- Carbine
- Block pinned
- 1/2×28
Price on requestBBL-GC-B103-MRP
// 4 in stock
6mm Max® — Mos-Tek and SOLGW
Six-millimeter reach on the standard 5.56 bolt face. A 10.5″ Mos-Tek for A5-equipped lowers, and the SOLGW Precision SPR at 16″ and 18″ — the 18″ also with the RIFLESPEED Gas Control. Coming soon: the Shaw Barrels 16″ 6mm Max® barrel (1:7.5″ twist, HBar contour, spiral fluted) — the barrel in our 16″ 6mm MAX SOTM4 complete upper — converted for the LMT® MRP™.
Mos-Tek
Mos-Tek 10.5″ 6mm Max®
10.5″ Mos-Tek precision barrel in 6mm Max®, built for BC Precision Ballistics and converted for LMT® MRP™ upper receivers. Six-millimeter reach in a CQB envelope, on the standard 5.56 bolt face.
- 6mm Max®
- Carbine
- Block pinned
- 5/8×24
Price on requestBBL-6MAX-B105-MRP
SOLGW
SOLGW Precision SPR 16″ 6mm Max®
16″ SOLGW Precision SPR barrel in 6mm Max®, converted for LMT® MRP™ upper receivers. 416R stainless, P3 polygonal rifling, intermediate-length gas with the block installed and pinned.
- 6mm Max®
- Intermediate
- Block pinned
- 5/8×24
Price on requestBBL-6MAX-B160-MRP
SOLGW
SOLGW Precision SPR 18″ 6mm Max®
18″ SOLGW Precision SPR barrel in 6mm Max®, converted for LMT® MRP™ upper receivers. 416R stainless, P3 polygonal rifling, rifle-length gas with the block installed and pinned.
- 6mm Max®
- Rifle-length
- Block pinned
- 5/8×24
Price on requestBBL-6MAX-B180-MRP
SOLGW
SOLGW Precision SPR 18″ 6mm Max® with RIFLESPEED
18″ SOLGW Precision SPR barrel in 6mm Max® with the RIFLESPEED Gas Control installed and pinned. Twelve numbered gas settings, #887 plunger fitted and #917 included.
- 6mm Max®
- Rifle-length
- RIFLESPEED, pinned
- 5/8×24
Price on requestBBL-6MAX-B180RS-MRP
Coming soon
Shaw Barrels
Shaw 6mm Max® LMT® MRP™ barrels
Barrels from Shaw’s dedicated 6mm Max® line — the same Shaw barrels offered in our SPIRE 6 series — converted for LMT® MRP™ upper receivers. Lengths, gas systems and pricing to follow.
- 6mm Max®
- Shaw Barrels
- Coming soon
// 2 in stock
6mm ARC — SOLGW and Ballistic Advantage
A 14.5″ SOLGW and an 18″ Ballistic Advantage Premium Black Series, both with the RIFLESPEED Gas Control installed and pinned. 6mm ARC runs the 6.5 Grendel case head, so both take a Grendel-pattern bolt.
Ballistic Advantage
Ballistic Advantage 18″ 6mm ARC with RIFLESPEED
18″ Ballistic Advantage Premium Black Series barrel in 6mm ARC, converted for LMT® MRP™ upper receivers, with the RIFLESPEED Gas Control installed and pinned. #887 plunger fitted, #917 included.
- 6mm ARC
- Rifle-length
- RIFLESPEED, pinned
- 5/8×24
Price on requestBBL-6ARC-B180RS-MRP
1 in stock
SOLGW
SOLGW 14.5″ 6mm ARC with RIFLESPEED
14.5″ SOLGW barrel in 6mm ARC, converted for LMT® MRP™ upper receivers, with the RIFLESPEED Gas Control installed and pinned. #887 plunger fitted, #917 included. One in stock.
- 6mm ARC
- Mid-length
- RIFLESPEED, pinned
Price on requestBBL-6ARC-B145RS-MRP
// Compare
All thirteen, side by side
Every row links to its product page.
| Length | SKU | Caliber | Gas | Gas block | Threads | Muzzle | Stock |
|---|---|---|---|---|---|---|---|
| 12.5″ | BBL-MB-B125-MRP | 5.56 NATO | Mid-length | Integral block | 1/2×28 | Thread protector | In stock |
| 13.9″ | BBL-MB-B139CL-MRP | 5.56 NATO | Mid-length | Integral block | 1/2×28 | Thread protector | In stock |
| 14.5″ | BBL-MB-B145-MRP | 5.56 NATO | Mid-length | Integral block | 1/2×28 | Thread protector | In stock |
| 14.5″ | BBL-MB-B145CL-MRP | 5.56 NATO | Mid-length | Integral block | 1/2×28 | Flash hider pinned and welded | 1 in stock |
| 16″ | BBL-MB-B160-MRP | 5.56 NATO | Mid-length | Integral block | 1/2×28 | Brake fitted | 1 in stock |
| 16″ | BBL-MB-B160CL-MRP | 5.56 NATO | Mid-length | Integral block | 1/2×28 | Thread protector | In stock |
| 10.3″ | BBL-GC-B103-MRP | 5.56 NATO | Carbine | Block pinned | 1/2×28 | Thread protector | In stock |
| 10.5″ | BBL-6MAX-B105-MRP | 6mm Max® | Carbine | Block pinned | 5/8×24 | Thread protector | In stock |
| 16″ | BBL-6MAX-B160-MRP | 6mm Max® | Intermediate | Block pinned | 5/8×24 | Thread protector | In stock |
| 18″ | BBL-6MAX-B180-MRP | 6mm Max® | Rifle-length | Block pinned | 5/8×24 | Thread protector | In stock |
| 18″ | BBL-6MAX-B180RS-MRP | 6mm Max® | Rifle-length | RIFLESPEED, pinned | 5/8×24 | Thread protector | In stock |
| 18″ | BBL-6ARC-B180RS-MRP | 6mm ARC | Rifle-length | RIFLESPEED, pinned | 5/8×24 | Thread protector | In stock |
| 14.5″ | BBL-6ARC-B145RS-MRP | 6mm ARC | Mid-length | RIFLESPEED, pinned | — | Thread protector | 1 in stock |
// Custom Shop
Finished to the build.
Every barrel ships shooter ready, with the gas tube installed and the SOT Standard Barrel Thread Protector fitted — CNC-machined steel, black or nitride finish. Two barrels are exceptions, and we have one of each: the 14.5″ chrome-lined Monobloc® ships with a HUXWRX Flash Hider-QD 556 XL pinned and welded, and the 16″ Monobloc® ships with a Rugged Suppressors MB011 3-port brake, timed and secured with Rocksett®. Otherwise no muzzle device is fitted unless you choose one — we carry muzzle devices from a number of manufacturers and can install and time them for you. The Custom Shop also gauges a headspaced bolt group to your barrel and quotes anything outside the cataloged configurations.
// Your barrel
Already have a barrel? Send it in.
Send us the AR-15 barrel you want to run — any length, any caliber — and it comes back converted to fit your MRP™ upper. $135 per barrel, about two to three weeks. Start with a conversion request; nothing ships until we confirm.
// Before you order
Fitment. 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 length. A barrel under 16″ on a rifle-configured firearm 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 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 CA, CT, DE, DC, HI, IL, MD, MA, NJ, NY, RI or WA, or to Denver, CO, or Cook County/Chicago, IL. Monobloc® barrels are HBAR-marked and can ship to Maryland.
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.
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.