You're saying the firearm rules are odd and incomplete? Well I never! There are rules for miniguns saying only certain weapons can be constructed as miniguns, but no rules for modding minigun on. What does that refer to? Presumably, there were high hopes of a system for making your own gun from scratch that never materialized.
Splitting your fire like that also costs you your smartgun bonus, so that's another 2 dice. You should definitely not do that, because you're essentially trading long/short for long/long with -3 to hit on both, which is bad.
Under those house interpretations, don't bother with SMGs. Or B&Ps, actually, since there's no need to use a F weapon to eke out a bit more RC when you don't need slots for HV. Use Ares Crusaders (more ammo, cheaper, and R), slap on Underbarrel Weight, Firing Selection Change (Full Auto), Rigid Stock accessory, Personalized Grip, and Barrel Reduction, then get Chameleon Coating later.
That also makes shotguns go from "niche" to "fuck it," if you need to kill multiple people, just use HV fire from your battle rifle.