Large Lasers and LB-5Xs both count as main guns in my mind, due to the combination of mass, damage, and range. ERLLs moreso even than regular LLs. This is especially true when your primary target is ASFs, which have to make a control check when they get tagged.
And since LB-5Xs are subject to the same damage boost as their heavy brethren when paired with advanced SLDF neurohelmets and an Augur Array, a refitted Rifleman with those four guns, theoretically does 32 damage a round out to just beyond PPC-range.
That’s pretty seriously impressive for a 60-ton ‘Mech.