Add an upgrade to them/entirely new pistol that gives a 15 round clip and an MBS of 1500 for soldiers, and 2000 for advies. Pump up the equip reqs for the new weapons as you see fit.
These changes should make the advy pistol far more effective in PvP - and give advies a respectable, while not crazy, PvM DD boost - while not effecting soldier PvP that much because PvP soldiers will still be rocking Dsharks and pistol soldiers still won't have much AR, anyway.
A well-equipped pistol/KMP soldier ODs a well-equipped Ofab Shark soldier under virtually any conditions, and while it would give pistol/KMP soldiers even more DD than before, it's not like they're going to be put into the same class as shades. Perhaps to further balance it out, put an HP or evade debuff on the soldier gun so the soldier becomes stronger in it's strong category and weaker in it's weak category. DO NOT do any such thing to the advy gun, and don't make the debuff as crazy as that found in the berserker implant.
Yeah, that's my idea. Let the whines about overpowered soldiers and how melee advies being fine = ranged advies deserving nothing commence.