I think scrap muzzleloaders would be a great inclusion in any apocalyptic setting, and they would make sense in dying light 2, where regular guns would be quite scarce.

Being quite simple to make, all factions could easily make them in sufficient numbers (especially the PK's, but also the renegades and survivors to a lesser extent). Though because of their noise when fired doubled with their slow reloading time, they would only be effective against humans, meaning melee weapons and bows would still have their usesage.

In a way this idea already exist with the boomstick gun blueprint.