I'm currently reading "A Rock and a Hard Place", the relatively new Grey Death Legion novel. In it, there are several references to firing only part of an SRM or LRM rack's capacity. For example, an Archer fires 4-6 missiles at a time to conserve ammo during a prolonged combat and a Commando fires a single SRM to kill a sniper.
This would obviously make tracking ammo more complicated, but I'm curious if anyone has tried anything like this in a game. (Or maybe it's an optional rule hidden in one of the books and I just don't remember.)