I spent most of my hunting years shooting 12 ga. slugs- the hollow-based- Foster slugs. I would think that IF YOU CAN find a lead slug designed for shooting in a .72 caliber rifle, a 3 dram load of 2Fg powder would be all you want to fire with that heavy slug.
A 12 gauge round ball weighs approx. 1 1/8 oz. or over 500 grains. I don't know any critter than walks on the N. American Continent that will not fall when hit by such a heavy, and large diameter RB. A RB made of pure lead will expand faster in flesh and bone, than even a Foster slug of the same lead. The hole punched through internal organs, and out the other side is Enormous, placing great shock to organs, and central nervous system, while causing immediate, and large amount of hemorrhaging internally and externally.
The weight of the gun, vs. the recoil of a 12 gauge slug is going to be the limiting factor as to what distance you can accurately shoot that slug, even out of a rifle. I know that there are groups of slug shooters who are regularly target shooting off rests with these large slugs at 200 yds. But they are using heavy barreled( usually 10 gauge barrels with a 12 gauge bore in them) to add weight up front, and wide butt stocks to distribute the recoil over a larger area of the shoulder. NO ONE is trying to shoot these same loads at that distance out of a double rifle, to my knowledge.
This is America, the land of the Free, and home of the Brave. You are certainly entitled to make up and shoot anything you want, no matter how much it hurts to do so. I support your right to do it, too. But, I have to ask why you would want to subject either the gun, or your body to the heavy recoil that such a gun will generate??? Think about it. I don't expect an answer- its a rhetorical question, instead. :surrender: :hmm: :thumbsup: :thumbsup: :hatsoff: