tenji:
Welcome to the Forum.
I think Bakeoven Bill is on the right track.
The recovered patches are the key to finding out the cause of an open pattern like you describe.
The recovered patches will have the entire outer edge tattered but the circular area where the ball was squeezing the patch against the bore should be in good condition.
If that area of the patch has cuts or burned areas it can indicate several different problems.
Cuts which will look like slits running in the direction of the rifling can be caused by the sharp edges of the rifling grooves.
They can also be caused by the sharp edges of the rifling grooves at the crown of the muzzle which can cut the patch as you are starting the patched ball into the bore.
Burn thru's are caused by either too hot of a powder load (not in your case), a lack of lubrication or a patch/ball combination that does not provide enough pinch on the patch.
I have shot some "pre-lubed" patches that were old when I bought them at the gun store.
The fired patches were not cut or burned. They were totally blown apart!
If your patches are blown apart (whether "pre-lubed" or not) it indicates the material is weak and if that is the case they will never provide good accuracy.
Next time you shoot your rifle be sure to collect all of the fired patches. They will be about 10-25 feet downrange and if it is slightly windy they can be blown up to 10 feet to the side as well.