Don't have experience with the '42, or .69 calibre minnie rifles, but have done a fair amount of shooting with the .58 calibre rifle-muskets.
Actually, a tight-fitting minnie will be most accurate...when used in war, the Army used loose fitting minnies so that hundreds of rounds could be fired, and a certain amount of accuracy was traded off for that.
To get your six inch groups or better, you will want a minnie that fits the bore nice and tight. And there's nothing wrong with a six-inch group, that's more than close, right on target, whether the target is a man, six-inch bull's eye, deer, bear or Elk!
Rat