Its not made to shoot a roundball...Especially a patched round ball...those guns are made with deeper rifling and a slower twist...That being said...I have heard of guys shooting a .45 caliber round ball in a sabot with reduced charges for plinking, small game hunting, etc...I have killed deer with a flintlock and a .45 caliber... But I was shooting 75 grains of FFF....
There are a ton of things you can try...First are you cleaning between each shot?? In a inline I do, with sabots its hard to not only load the 2nd shot into a dirty barrel, the 2nd shot usually doesn't group as well...With muzzleloaders you are hand loading each shot, so I prefer to sight in with a clean barrel...
Also...There has been alot of improvements in sabots the last few years, are you using the sabots that come with the bullets?? Or buying bulk...MMP makes a thiner sabot (I think its a MMP-24) and Harvester has come out with a Crush Rib sabot to make loading easier...You could try going with a .44 diameter bullet (the Hornady .240 grains) with the green sabot, this will make loading easier...Or you could even go with PowerBelts, while some guys don't like them because they expand easier especially with higher powder charges, I do know some guys that use them and kill deer with them..
In addition...I prefer loose powder so you can taylor the charge to what your gun like...My Knight Disc prefers 90 grains of Pyrodex over 100 grains....The group size goes from about 2 1/2 inches at 100 yards to one ragged hole....
What have you tried so far??? Are you just fighting the tight bore or is accuracy poor as well...What type powders have you tried...Is it tight with a clean bore???
Finally...I have lapped a few bores that were tight, this might help, especially if the bore has tight and loose areas in it...