Not familer with your gun but I can tell you this... Look at the barrel muzzle first and see if there are any "dings" or "dents" right at the inside of the muzzle or I should say on the muzzle opening. One of the biggest reasons for inaccuracy in muzzle loaders is caused by the ball (bullet) hitting a small dented area on the muzzle as it leaves the barrel and that causes the projectile to wobble in the air.The dents or dings happen during the loading process from the starter rod or the loading ramrod. The cure is to have the muzzle chamfered by a good gun smith.
Next thing to do if the muzzle is alright is to take down the front site with a bastered file a little at a time until you hit right at the height you want at 50 yards. Next thing is to bend the front sight slightly to the left. this will cause you to move the barrel right to be on target.
6" at 50 yards is allot so I would lean more to the muzzle problems on this one. You may need a good magnifying glass to see any dings or dents. It takes very little to through off your accuracy.
I had this problem with a 50 cal. Hawken years ago and I couldn't hit anything with it. After the muzzle was chamfered I ws hitting 3" circles at 70 yards when I was shooting good
Mike