The numbers just don't work.
Consider that both have a measly damage fall-off range of 3. For comparison the fireaxe has a range of 2.9. Shells max range = 18, slugs = 25.
I believe it works as a linear drop-off of 100% - 0% damage from damage fall-off range - max range. Correct me if anyone knows otherwise. Edit: Corrected in next post
Given that, shotgun slugs begin out-damaging shells if all pellets hit (and again that spread is fairly tight) at a range of 9 at which slugs do 58 damage to the shells 57.
But here is the main issue...at a range of 9.7 slugs drop to the same damage as the hunting rifle, and at 11 to same damage as the sniper rifle. The rifles don't even begin losing damage till range 60/70 respectively. So you have a very small zone at range 9 - 9.7 where slugs should out-damage both...
I mean if you find them in the wild, cool, use them. But especially in terms of crafting, shells do more damage at close range and 7.62 does more damage at medium-long range for much fewer resources. Slugs are objectively just less efficient resource wise. I would concede that at close range aiming for headshot you may miss a pellet or two, but it still isn't enough damage to justify the roughly 3x higher resource cost.
At a bare minimum, they need to increase the effective range or the damage of slugs for me to ever use them over shells or a rifle.