The L85A1 is a conventional steel made, gas-operated rifle locked by a rotating bolt engaging in lugs behind the breech and carried in a machined carrier running on two guide rods; a third rod controls the return spring.

The gas regulator has three positions, a normal opening for most firing, a large opening for use in adverse conditions and a closed position for grenade firing. The trigger is in front of the magazine and there is a long connecting rod running to the mechanism. On the left side is a selector lever that is set either to semi-automatic or automatic fire.

The L85A1 IW can be fitted with a high-performance optical sight which enables the weapons to be used under poor light conditions and is also useful for surveillance.

An emergency open sighting system is also included, being permanently fitted to the body of the primary optical sight. In the British Army the optical sight is issued only to front-line combat troops, other users employing `iron' sights consisting of a double aperture rearsight housed in a carrying handle mounted on the dovetail sight base, with a foresight blade on the gas block.

The L85A1 IW was designed to be simple to dismantle, without special tools, into the main subassemblies for cleaning and maintenance. The trigger mechanism is a self-contained assembly in a pressed steel housing which is located to the main weapon body by two pins and a small butt-plate. The main body is a steel pressing which houses the bolt and carrier assembly and guide rods which locate in the barrel extension welded into the body and into which the barrel is screwed.
A number of accessories are available including a sling, bayonet, blank-firing attachment, cleaning kit and a multipurpose tool. An adapter kit can be fitted to allow the firing of 0.22 LR rimfire ammunition.

Electronic individual weapon
As a pure research project, British Aerospace, Royal Ordnance Division and the Defence Evaluation and Research Agency (DERA) at Fort Halstead, have combined to investigate the introduction of electronics and power supplies to small arms systems. The project involved the use of a modified L85A1 IW and introduced a programmable electronic firing system with features such as changeable rates of fire, status reports, and so on. Operating the trigger actuates a microswitch to start the firing sequence. The project required special electrically initiated ammunition using bridgewire technology in the cartridge primer. Power for the electronic unit is housed inside the pistol grip. The program is still in progress.

Specifications
Individual Weapon
Cartridge: 5.56 × 45 mm NATO
Operation: gas, selective fire
Locking: rotary bolt, forward locking
Feed: 30-round box magazine
Weight: weapon without magazine and optical sight, 3.8 kg; with loaded magazine and optical sight, 4.98 kg
Length: 785 mm
Barrel: 518 mm
Rifling: 6 grooves, rh, 1 turn in 177.8 mm
Trigger pull: 3.12-4.5 kg
Muzzle velocity: 940 m/s
Rate of fire: cyclic, 610-775 rds/min