Yen Lo-Wang "Shadow" (1xLL, 2xAC2, AMS, XL295)

AC/2's have the highest DPS per ton of any ballistic weapon (except the machine gun, which we wont go in to).

In fact it has twice as much DPS per ton as the AC/20. Fitting 2 of these results in better range, DPS and leaves 2 tons spare for extra ammo.

It doesn't suit my play style, but it works if you can aim.

 

That's exactly the reason why 2 AC/2s are so great on medium mechs (if DPS weapons fit your playstyle).
With 3 or more autocanons the DPS per ton is a bit lower, because you need additional heatsinks to make it work efficiently (one of the reasons why I like the 2xAC/2 + AC/5(s) combo instead of 3 or more AC/2s).