Narrow Barrel > Galvanized/[Fatal Acceleration], though projectile speed helps, but it won't help much if the projectiles don't land in the vulnerable spot.
DoT from electricity helps proc [Narrow Barrel] for Hemocytes, so don't put a toxin mod in there.
Shotgun Spazz would be the best option, but I don't want to throw in another 1 or 2 forma.
The reduced recoil from Primary Deadhead would help, but feel free to use Primary Merciless instead.