I DO load my own specialty shells! Among them .725 round balls, pumpkin balls as they are called in the US if I am not mistaken. I also load the air pellet shaped full bore .72 cal projectiles that weigh in excess of 700 grains, and buck & ball.

Sadly I think the barrels of this shotgun are too thin to rifle them to turn them into a paradox. I always toyed with the idea of using rifled chokes but so far I have not acquired a shotgun suitable for it.

I do have a small single shot break action shotgun that shoots the balls into a palm sized group at 50 meters even without rifling, but oh boy it kicks the everloving **** out of me. My most accurate load is that 540 grain roundball at 1600 fp/s, but said shotgun weighs around 5.5lbs or so and has a 20'' barrel. But it does have rifle sights.
It recoils worse than a 500 nitro express and I'm always afraid of breaking the stock.

Reminds me, I've always wanted one of those H&R break action shotguns with a heavy rifled barrel but it is impossible to get them here in Europe. We do not have shotgun seasons so nobody is interested in them.
That thing would be the holy grail for me.