A few thoughts:
Many melee-AT APPs add control powers, like a single target hold or a cone/AoE immobilize. Take suppressive fire from dual pistols, drop the damage down to single hold levels, and give it a hold effect, (treat it like the cryo ammo version if that's simpler). Take empty clips and call it something like "pin-down" and turn it into a cone immobilize, with the damage and recharge adjusted accordingly. Add a targeting drone/focused accuracy-like power, (maybe call it sight-in or something appropriate). You can then add bullet rain for the AoE damage power, and hail of bullets as the capstone power, (maybe lower the damage, keep the +def when using standard ammo, and perhaps even add a -res effect).