Ooh, here's an idea. 12 gauge slug ammo for all shotguns. +10 to range and it has mild armor-piercing capabilities, but is a bit harder to aim with.
So the default buckshot looks like: 1/1, 0 DR, -20 AC
And slugs would would like: 1/1, -20 DR, 0 AC, Extra range
I'd imagine this "extra range" perk could work similarly to the "penetrate" perk on AP ammo.