Fragmentation rounds are basically bullets designed to turn SP weapons into shotguns and their mechanic is a decreased RoF in exchange for Storm. I have to say that giving shotguns storm is a fine representation of hitting a target hard without unnecessarily complicating them. Make the damage high enough and the range short enough and the end result will be about right in my opinion.
It's too easy to hit with Spray
Worth a try, if damage is increased to the old value of 1d10+4 again.