MRMs and light gauss rifles are generally regarded as pretty poor weapons. The former because they have a terrible short range bracket (0-3, same as SRMs) and the latter because they weigh almost as much as a gauss rifle but do roughly half the damage.
I suggest some simple unofficial rules be put added so that people may want to use these weapons more often.
MRMs : Change short range bracket to 0-5 hexes, giving them the same range brackets as other medium weapons such as large lasers and AC10s. This also allows more efficient use of c3 as they can spot at 5 hexes instead of 3.
Light gauss rifle : Change damage from 8 to 11. Not sufficient enough to headcap mechs, but large enough of a increase to make them more dangerous on the battlefield as opposed to a long range large laser.