I would suggest spending an afternoon shooting different powder/patch/ball combinations keeping detailed and copious notes. I went from a rather lackadaisical four-inch group down to 'touching cloverleafs' using this method with my 1:48 twist Lyman Trade Rifle and discovered it likes .495RB, .015 patch and 70gr Pyrodex RS better than anything else I tried.
I was glad I kept the notes though, as time makes you forget your hard work quickly!