The key control for each laser is shown next to the weapon entry in the bottom right of view. Each key is bound to a weapon mount and each weapon mount has it’s own traverse limitations. The starting mech has laser on left arm (Q key) and minigun on right arm (E key). The canon is top mounted (2 key) and does not traverse down. The canon is good for long range and the minigun for short range. The laser is admittedly hard to use, I should probably have removed it.
Later you get mechs with shoulder weapons (1 and 3 keys) which have very narrow horizontal traversal but are good for medium and long range shots.