Your gun should be capable of doing much better than a 5" group at 100 yards provided all is tight, you are on a good rest, are very steady, can execute a smooth trigger pull, the bullet/powder load is one the gun likes, etc. If all else is good, then do a little experimentation to find the bullet/powder combination that shoots best in your gun.
Change to different bullets, sabots, etc. and see if any shoot better than the others. Note, a loose fitting easy to load sabot/bullet will not likely be the most accurate. It is usually the tighter fitting sabot/bullets that will shoot the best.
Try different powders and amount of powder. Some guns shoot better with more or less powder. You can also try different primers. Some are hotter than others and do effect accuracy.
There is some very good info on the Modern Muzz website at: http://www.modernmuzzleloader.com/phpBB/
Go there and click on "Inline Muzzleloading" or "New To Muzzleloading" and you can read alot of good info about the gun, loads, shooting for accuracy, etc.
Check out the info on the new Blackhorn 209 powder. It is said to be the best and cleanest burning powder for a muzz gun. It is a loose grain type powder and while it is not as convenient as using the pellets, it does allow you to vary the powder load in small increments to achieve best accuracy.
It sounds like you just need some range work with a good rest of sandbags or similar and some experimentation with differnt loads to find what your gun shoots best.