Although I didn't even know Sears sold a muzzleloader, and I don't know where your instructions are (did you try your sock drawer, my wife sticks stuff in there when I'm not looking), I think we need a little more information about the gun.
Is it a side-lock precussion style (like the old guns) or is it one of the Modern style rifles with a bolt or break open breech?
Miroku builds pretty good guns (usually very good). I'm surprised the trigger pull is so heavy.
If you can fill us in on the lock style, and the barrel diameter we can make some good guesses.
If you can tell us what the rate of the rifleing twist is we will be even in a better position to give you some good answers.
If you don't know the twist rate but have a .45 cal cleaning jag, a patch and a cleaning rod you can determine this by wrapping a piece of tape around the rod 2 feet from the jag.
Just start the patch and jag into the muzzle and mark a line (with a thin felt tip marker) in line with the sight on the rod behind the tape. Carefully push the rod/patch/jag down the bore allowing the rifleing to twist the rod. When the 2 foot tape gets to the end of the barrel, notice how far the "sight" mark has turned from where it started (in line with the front sight).
If you tell us it went from the 12 o'clock position to the 6 o'clock position (1/2 turn) the twist is 1:48. If it went from the 12 o'clock position to the 4 o'clock position (1/3 turn) it is a 1:72 twist.
Don't worry about what the actual twist is, just tell us where the mark ended up.
If we know the twist we can tell you what kind of projectile will shoot best in it.
