All Battlemech locations which contain missile hardpoints have a maximum number of "tubes" in them; you can't fire more missiles at one time than you have total tubes. So, if I have (10x2) tubes on a 'mech's torso, I can fire 20 LRMs at once from that Torso, but 30 (via two LRM15s, for example) will fire twenty in the first volley, and the last ten about a half-second later. This is important, because your launchers' weapon cooldowns don't start to come back up until they have launched all their missiles - and because two smaller volleys of LRMs give the enemy more time to evade, and are more vulnerable to the enemy's AMS.
Edited by Void Angel, 11 October 2013 - 02:19 PM.