TRW LMR - Low Maintenance Rifle (USA)

TRW LMR (Low Maintenance Rifle) in its final shape, right side, with attached M6 bayonet

TRW LMR (Low Maintenance Rifle) in its final shape, left side, with 30-round magazine attached

Caliber: 5.56x45mm M193
Action: Gas operated, locked by rollers
Overall length: 870 mm
Barrel length: 493 mm
Weight: 3.63 kg loaded
Rate of fire: 450 rounds per minute
Magazine capacity: 20 or 30 rounds

The Low Maintenance Rifle project was initiated by US Army as a result of field experiences, gained through Vietnam war. The standard issue M16 rifle was found to be very sensitive for improper maintenance, and it was decided to look for an infantry rifle, which could work with much less attention paid to routine maintenance, cleaning and lubrication. The one possible niche for such weapons was the "insurgency weapon" for various pro-US (or anti-communist) movements in the third world countries. Development of the LMR project was handled to the US company Thompson-Ramo-Wooldridge (TRW) Inc, and work commenced in 1971. Several prototype rifles were built between 1971 and 1973, when US Army decided to stop funding for LMR, as the Vietnam war was over, and M16A1 rifle was considered to be mature enough.
It must be noted that at least one LMR prototype was built for 5.6mm XM216 ammunition that fired saboted sub-caliber flechette projectile. However, most of LMR rifles were built for then-standard 5.56x45 M193 ammunition.
Required "low maintenance" was achieved by using new and improved corrosion-resistant coatings for all metallic parts, as well as thin layers of dry lubricants applied to all moving parts. The number of parts was also decreased, compared to the M16 rifle. TRW LMR was simple in use and could be disassembled and re-assembled without any tools.

TRW LMR is gas operated weapon that uses long-stroke gas piston, located at the right side of the barrel. Gas port is located near the muzzle, and gas block serves as a base for front sight. The barrel is enclosed in tubular metallic handguard with oval cooling slots. TRW fired only in full automatic mode, and from open bolt. Locking is achieved by a pair of rollers, that engage respective cuts in the barrel extension when in battery. Unlike most other roller-locked designs, LMR bolt had rollers at the top and the bottom, not at left and right. Charging handle was attached to the gas piston rod, and when turned down, locked the bolt in forward position for added safety. Trigger unit and pistol grip were borrowed from US M60 machine gun; manual safety was located at the left side of pistol grip. Magazine (of standard M16 pattern) was inserted horizontally from the left; empty cases were ejected to the right through the ejection port, which has dust cover that opens and closes automatically as gun is fired. Since the LMR was built into so called 'straight' or 'inline' layout, open sight were put onto high bases. front sight was of post type, rear sight was a flip-up diopter with two apertures. LMR combat capabilities could be further enhanced with addition of light, removable bipod and M6 bayonet, which was attached at the top of gas cylinder, to the right of the barrel axis.