Since I tried it, I figured I'd share my results. The roundballs were Hornady .36 Cal(.375" dia) roundball weighing 80 grains. The caliber is .375-338 (aka 375 Taylor.) Fed 210 primers were used with 10.0, 11.0 and 12.0 grains of Unique. Cases necks were expanded and flared with a .375 Win die and roundballs were seated with finger pressure only. A light roll crimp used to hold the balls in place.

After punishing myself will a final sighting in with full-power 300 gr partition hunting loads, I decided to take a necessary break. These hunting loads are at about my limit for recoil - 300 grains at 2,500 fps. I was sighting in with an aperature sight systems at 40 yards. All roundballs hit paper at 40 yds using a 25 yd pistol target. Not a great group, but hey, they hit the target. Felt like a .22 WMRF is terms of recoil. Would be great for teaching my kid how to shoot the cannon.

I ran the 12.0 grain loads over the chrono. 1,265 fps average. 3 shots total. SD = 87 fps. Extreme spread = 171 fps. Not sure how to improve results - i.e. reduce the extreme spread and SD. I know I should not expect much, but 2.0 to 2.5" at 25 yds would be nice. Should I be using pistol primers instead of rifle primers? I have a 1:14 twist. Aren't muzzleloaders (at least ones that shoot roundballs fairly accurately) something like 1:30+ twist? Any accuracy tips for the roundballs would be appreciated.

Lars.....use of large pistol primers like the Remington 2 1/2 are appropriate...you might also bump the powder charge a bit to see if that helps.
Depending how "hard" the balls are you might want to coat them with something so they don't lead up your barrel.....you didn't mentioned how many balls you used?