It is designed to fulfill the role of the squad automatic weapon. Like the LWRC SRT, the LWRC IAR utilizes a gas-piston design in lieu of the AR's direct impingement design. It was developed for the United States Marine Corps Infantry Automatic Rifle program, which seeks to replace the M249 SAW with a lighter, more reliable and durable 5.56mm weapon.
The weapon fires from a closed bolt in semi-automatic mode, and from an open bolt during full automatic mode which is labeled as "OBA" for Open Bolt Automatic. While in OBA mode, the first round may be fired from a closed bolt (it will then lock back and subsequent shots will be from an open bolt until the operator manually closes the bolt again).
Firing from an open bolt increases cooling and eliminates the potential for accidental discharges due to rounds "cooking off" in an overheated chamber. It also allows for a faster rate of fire. However an open bolt design means that the first round fired will have reduced accuracy when compared to a closed bolt design. This is due to the fact that when the trigger is pulled, the bolt slams forward under spring tension, stripping a round from the feeding device, chambering it, then firing it. This sequence of events shakes the firearm and takes a (possibly crucial) split second longer than a closed bolt design to fire the first round (greater lock time). This also introduces extra potential points of failure in the ignition of the first round.
The IAR's ability to fire from both modes allows the gunner to deploy the weapon in OBA mode with a round chambered on a closed bolt. This allows the gunner to fire his first round as accurately, reliably, and quickly as a rifleman, while following that initial round with Squad Automatic Weapon (SAW) level of fire
The plan is to have the USMC buy 4100 IARs to replace 2000 of the Corps SAWs. This will reduce the number of SAWs in the marine corps from 10,000 to 8000. The remaining 8000 M249s will be kept in service, as the M249 will not completely go away in the USMC. The Army has ruled out the option of replacing the SAW with the IAR, because it would result in a loss of fire going from 200 round belts to 30 round mags, and instead is looking into a new belt feed light machine gun such as the MK 46 LMG.