I don't think that you're being overly cautious.
You know the gun better than we do and you're the one shooting it and taking the risk, if any.
I wouldn't want to shoot an .80 caliber lead ball out of it either.
Balled up newspaper can also be used for wadding both over and under the shot if it's tightly compressed on top of the powder, tight enough to seal off the hot gases when fired. [Any paper can be used really, such as copy paper.]
It's a good idea to use light weight projectiles or small shot load since that will help keep the pressure down.
I would hate to see the bore get damaged by shooting steel, although some folks do shoot steel BB's or small caliber slingshot ammo. But that's usually in a modern gun made with a modern steel barrel.
An .80 lead ball would weigh about 800 grains [or 1.8 ounces] compared to an ounce of lead which weighs about 437.5 grains.
Right now .22 ammo is pretty cheap at only 3-5 cents per round or so. I've shot .22lr bullets that were pulled from misfires and loaded them into smooth bores.
Round nose .22lr bullets usually weigh 40 grains each, and the hollow points about 36 grains each.
If you can get a hold of some .22lr ammo, the bullets can be easily popped out or wiggled off using pliers. But don't crush the priming compound in the rim of the case
Then you can shoot some lead instead of steel nuts.
Just make sure that the bullets stay on top of the powder and that the powder is completely sealed off by tamping down just enough newspaper to seal the entire bore. and to keep the bullets in place on top of the powder wadding.
The .22lr bullets could probably easily be cut in half to double the number of pieces.
And keep track of how many total bullets that you load and consider to be a safe weight.
Starting loads are traditionally one grain of powder to match the caliber number of the bore size.
But 100 grains doesn't seem to be excessive for such a large bore.
Have fun and be safe.
