At 10 tons, the LRM 20 is the largest of the Long Range Missile family. Its indirect fire allows support 'Mechs to pummel the enemy from safe distances. The LRM 20 is capable of heavy damage at range but the large number of missiles fired at once means ammunition shortages are fairly common. An LRM's minimum range is 180m, below which the damage decreases to nearly nothing at point blank range. The LRM 20 is subject to Heat Scaling.
The Artemis IV FCS upgrade increases the tracking strength and decreases the lock down time by 50%. However, each launcher requires an extra critical space and 1 extra ton for the Artemis control system this makes Artemis on multiple smaller launchers less tonnage efficient than on a single larger launcher.
While ECM makes the effective use of LRMs slightly more difficult, it is possible. If you are alone with an ECM equipped foe, staying within 181 and 200m will allow you to use your weapons with lock. If you have a TAG spotter, as long as they are at least 181m from the target and maintain the tag on the target, you will have your target. If you have a PPC spotter, you have your target as long as they keep the ECM disabled.