How would one figure out the Hardness and HP of a gun? The closest thing I could think of is either a projectile weapon(5 and 5) but that doesn't seem right for those made of metal.
I believe the general intent is for early firearms to be fragile, which is why they can break just by using them, so I think projectile weapon still makes a good amount of sense.
5/5 or possibly 10/5 to account for being made of mostly metal. That is how I would rule it at least.