The NARC Missile Beacon is a missile that replaces the explosive warhead of a missile with a homing beacon. When fired at a 'Mech, the missile clamps onto the target and transmits homing information for friendly missiles. Unlike TAG that requires the firing 'Mech to maintain a Line-of-Sight on the missile target, the NARC Beacon allows a 'Mech to fire the homing missile and disengage the targeted 'Mech, allowing a more of a hit and run style missile spotting.
The homing signal increases the tracking strength of friendly missiles by 50%, allowing for more missiles to land on target. Additionally, the target lock will take longer to decay for the missile carrying 'Mech. The tracking strength of the NARC does stack with the TAG's tracking bonus, but does not stack with the Artemis IV FCS's bonus. The beacon will stay active for 20 seconds before the homing signal stops. NARC Beacon Missiles are an unguided missile much like SRMs, so care will be needed to land a successful hit on a target. This is especially important when considering the low number of missiles per ton carried by the Narc Beacon (12). A successful NARCing of a target will be indicated by a series of bars above the targets target box (pictured below).
NARC Beacons will counter ECM if it hits the source of the ECM. Targets that are NARCed but shrouded by another BattleMech's ECM will still be protected by ECM.
The NARC Beacon will also provide XP and C-Bill rewards. This reward is given to pilots that use NARC to mark a hostile, and that hostile takes damage from other friendlies. Given once per hostile 'Mech per match.
50% bonus to missile tracking strength
50% faster missile lock on time
Target lock takes 25% longer to decay
Min 1 ton of ammo needed (12 shots/ton).
Does not require LOS once attached to target.
Counters ECM while beacon is active.
Lasts 20 seconds.
Multiple NARC Beacons do not improve the bonuses to accuracy and targeting.
NARC effect HUD Icon
Provides the same bonuses to accuracy and targeting as the NARC
These bonuses do stack with each other
TAG requires a beam be held on target while the NARC requires you to hit your target with a missile
Using TAG and NARC together decreases weapon lock on time by 75% 
Using TAG and NARC together increases missile tracking strength by 75% 
Provides different bonuses than the Artemis IV FCS system
Artemis replaces the bonuses of the NARC, even if it does not have LOS