I would say buckshot because it's still technically a shotgun.
I would say slugs because the Jackhammer is unique looking, therefor should perform in a unique way.
I wouldn't say explosive rounds, because no matter how it performed, it would be designated the "noob weapon".
I voted buckshot. But I think it should be selective between buckshot and and slugs via /command. But since I know you hate making those, it will probably end up with slugs. Also, the new model is nice. The wood is a nice touch.