Barrels could be switched by removing two Torx head screws. I placed the carbine-length model on a full-automatic Colt M4 lower receiver and fired more than 300 rounds each through the 10.5-, 14.5- and 16-inch barrels—much of which was on full automatic—with no malfunctions. In addition to the barrels chambered in 5.56x45 mm NATO, LMT included one chambered for the .499 Leitner Wise, which at that time was being considered by the U.S. Coast Guard.
Last year, I contacted Lewis to request current-production rifles in both direct gas-impingement and piston operation for testing and evaluation. Both were on semi-automatic-only LMT Defender lower receivers, although the piston-operated rifle used an H2 buffer instead of a standard carbine buffer to prevent bolt carrier bounce and light primer strikes in burst fire. As of this writing the MRP offers chamberings in .204 Ruger., 5.56x45 mm NATO, 6.8x43 mm SPC and .308 Win., although others may follow.
LMT offers two versions of the direct-gas MRP: the CQB MRP, which is intended for carbine-length barrels, and the standard MRP, which is intended for 18- to 20-inch barrels. I received the CQB MRP (CQB16) with 10.5-, 14.5- and 16-inch barrels for testing.
The barrels had chrome-lined bores, and all had been proof-fired and magnetic-particle inspected to ensure they were free from stress fractures. LMT is no stranger to government testing requirements and protocols. Before a barrel is accepted for the MRP it must fire a 1.5-inch mean spread with lead-core bullets (not M855). Barrels come preassembled with gas blocks and tubes, making removal and replacement a modular affair.
The bolt carrier group on the direct gas-impingement carbine is an exclusive LMT feature. The carrier was re-engineered as an enhancement to the M4 to be tested by SOCOM. It has many changes from the standard bolt carrier, including five gas exhaust ports instead of three. The most important change is in the cam track, which has been altered so there is more dwell time before the bolt unlocks on the 14.5-inch M4 barrel. This slows the opening stroke, allowing pressure to drop inside the cartridge case, aiding extraction reliability and decreasing wear on the bolt. Also, four grooves placed on the bottom of the carrier aid reliability in harsh environments. The interior and carrier key were chrome-plated.
The redesigned bolt is of a stronger proprietary material (LMT wouldn’t divulge its composition), making it more difficult and expensive to manufacture. All bolts, whether enhanced or standard, are manufactured by LMT and are proof-tested and magnetic-particle-inspected. The lugs have relief cuts designed to allow them to better distribute the force that acts upon them during firing. The extractor, too, has been completely redesigned. A significantly more aggressive extractor claw, backed by two springs, grips the rim of the cartridge case.
The upper receiver incorporates a forward assist as well as a case deflector. The quad rails are marked to aid in placing the optics or accessories back on the same spot. All the barrels tested had the G.I. 1:7-inch twist rate with an A2-style compensator.
The upper and lower receivers fit together nicely with little to no movement. The selector settings were clearly visible and highlighted on both the left and right sides. The rifle came with three TangoDown rail protectors on it. Also included was a torque wrench that permitted the required 140 in.-lbs. on the two bolts that hold the barrel into the upper receiver.