Higher tier shotgun shells are downgrades.#955

Buckshot damage: 5 * 8 = up to 40 if all pellets hit.
Slug damage: 20.
AP slug damage: 30.

If you use the shotgun as a shotgun, fighting at close range, slugs and AP slugs are pointless to craft as they will hurt your DPS. For a longer range option the revolver performs better than the shotgun with slugs anyway.

That’s not a bug.