A patented AR-15-pattern is produced with a bolt design featuring a redesigned extractor intended to improve the extraction of cartridges under adverse conditions.

[2] The Monolithic Rail Platform (MRP)[3] was created by the company, a one-piece, Picatinny-topped AR-15-pattern upper receiver made from a forged aluminum block.

At ranges between 400 and 800 m (437 and 875 yd), the then available small arms capable of returning effective fire, were the general-purpose machine gun and the bolt-action sniper rifle.

These weapons chambered for the fully-powered 7.62 mm NATO cartridge were not well suited to increase the effective engagement range of the British eight-man rifle sections.

The LM308MWS was submitted for the British Ministry of Defence's Urgent Operational Requirement (UOR) for immediate deployment of a semi-automatic 7.62 NATO caliber sharpshooter rifle in Afghanistan.

The standard optic for the L129A1 is the TA648-308 6×48 Trijicon ACOG providing a 8 mm exit pupil for ample light gathering and a wide field of view.

[14][15][16] The ACOG is mounted to a Picatinny rail to which is fitted a Trijicon RM01 1 x Ruggedized Miniature Reflex (RMR) for Close Quarters Battle use.

[24][25] The gas-operated, rotating bolt (internal piston) rifles were delivered in May 2017, and soldiers began training with them at Waiouru Military Camp on 15 June 2017.

[31] The upper receiver has a Picatinny rail on the top and M-LOK compatible attachment points on its sides and uses a short-stroke gas piston system.

[40] These Swiss (professional) special forces units selected the Schmidt & Bender 3-20×50 PM II Ultra Short telescopic sight with the TREMOR3 reticle that enables quick targeting up to 800 m (875 yd) as their standard aiming optic.

[41][42] Within the Royal Marines at least, a MARS-H based L129A2 designated marksman rifle chambered in 6.5 mm Creedmoor with an 18 in (457 mm) barrel, a fully adjustable DMR stock and lengthened hand guard, a Leupold Mark 5HD 3.6-18×44 M5C3 Desert IR telescopic sight with TREMOR3 reticle, a HuxWrx suppressor, an Envision Technology ballistic calculator, and a Pixels-on-Target thermal sight was adopted in 2023.

[43][44] The main disadvantages of this change are introducing a new chambering in the logistics chain, and due to a higher Oratio is shortened barrel life.

[45] The upper receiver still uses a gas-operated, rotating bolt (internal piston) system like the L129A1, but the handguard attachment points on its sides were changed to be M-LOK compatible.