Part of me feels like the slug analogy was supposed to be a reference to real world gun slugs but neither of them are shotguns so it's a pretty loose connection. There is a full-auto shotgun based on the AK family, the Saiga-12, so it's still a tenuous at best.
The slug reference might be because of how slugs have sex. It's very weird and hard to explain (seriously just look it up online) but they get intertwined and tied with each other.