A handy tool for fixing weapons mathematically is SJ's
Weapon Stats.
The primary problem with stock weapon balance is that the damage ratios of high level APBs (and for their part MBs) are way too high in comparison to everything else. The special advantages of pretty much all of the non-mainline weapons (DUC/MB/APB) are overwhelmed. :-\ Features like high damage-per-hit for WMGs, Torpedos, etc. just don't have enough oomph compared to the massive damage loss suffered by not using these weapons. Part of the problem is lackluster defenses that don't really provide much necessity for high-damage shots, outside of Crystalline Armor. But even that is not a problem for the APB (esp. with some Shield Depleters..), when you factor in the insane stock mount setup that simply multiplies damage exorbitantly without any drawback.