One of my friends had an old black powder double rifle. Muzzle loading cap. Can't remember exactly the caliber but it was just under 60...57.58...something like that. Huge bore...looked like a shotgun. Seems like the bullet weighed in somewhere between 690 and 720??? We took it out hog hunting once. Didn't see anything but we shot it down a creek bed...probably 100 yards plus a little.
It was the funniest thing. You had to aim at the moon and you could just about reload the thing before that hunk of lead hit downrange.
We desperately wanted to shoot a big ole hog with it just to see how many times it would roll it over.
We were checking the velocity of the bullet with my chronograph...boom nicked the chrony...friend profusely apologized...we set it up again as it was still working...boom...that was the end of the chrony. Dead hit. LOL
I agree with you...a double rifle would be one cool shooting implement to add to the collection.