Given how utterly cheap shotgun ammo is, I think that the 2 uranium per shell is fine. Consider that a shell of ten shots is somewhere in the ballpark of equivalence to ten ammo magazines. Also consider that the combat shotgun with piercing shells is not on the same tier as the submachinegun with piercing ammo, rather, matching that submachinegun tier with a combat shotgun would be done with basic shells. Piercing shells were already a step ahead of piercing rounds. I don't know if it has been changed at any point, but the current damage is:
Uranium ammo: 24 damage
Uranium shells: 12 damage, with 20 pellets
Using the combat shotgun, the base damage is 14.4 with a base fire rate of 2/s. This puts the shotgun at a base DPS of 576, more than twice as high as the submachinegun at 240, and with quite a bit cheaper ammo (including uranium cost). I think this balances well with the spread of the weapon making it difficult to concentrate fire into a single target, meanwhile the much lower per-hit damage means you're losing a lot more to the physical damage resistance of big and behemoth biters in particular.
I actually prefer having more pellets rather than more damage per hit, because this diversifies shotguns away from submachineguns to a greater extent. But earlier shells need fewer pellets with more damage per pellet instead, due to the damage mitigation of biters in particular.
It seems pretty well balanced to me.
