Baldman,
At 25 yards, it doesn't get much worse than 6-7 inch groups. At 100 yards, you can multiply that by 4 and you have a 24 to 28 inch group. If you can call that a group.

If you are consistent with your loading, and your barrel is not excessively fouled from shot to shot, then I would say that your gun does not like the Maxi-Hunters.
My dad has a T/C Hawken that shoots a 385 gr. Buffalo Bullets very well. My T/C Hawken hates them.
My groups never got really tight until I learned to seat my bullet with the same amount of pressure every time. From measuring the powder, to seating the bullet try to do it exactly the same way every time.

Try several different loads. At 25 yards it usually doesn't take long to start making jagged holes in your target with a muzzleloader.

C. Davis