The issue is that when you switch to slug ammo, while the damage is consolidated into one projectile, when the slug round gets a critical it only does extra damage equivalent to 1 pellet from a 7 shot round.
So if say you have a shotgun that does 10 damage per pellet with a 7 shot load, and 10 damage per crit, with a slug you have 1 projectile dealing 70 damage, yet you still have a measly 10 damage bonus on a critical hit instead of an equal bonus of 70 damage.
My thought is to add an effect to slug ammo that multiplies the weapon's base critical hit damage by 7, so the slug does proper critical hit damage.
I tried to do this myself, but there isn't an ammo effect I could use to do this, so It is beyond me. I am hoping that a modder somewhere knows how to implement this.
Also, if it works out maybe the same could be done for the other shotgun shells that shoot less than 7 pellets, which would require GRA as that is the DLC that adds 4 pellet shot shells, and flechette shells with 6 projectiles.