At the risk of FtDH (Flog the Dead Horse), I think Dan got it in one. Remember that this is, at the end of the day, a resonant mechanism, and that the vibrations of the barrel will be effected by any of a number of factors, including (f'rinstance) where the lugs are placed, and how snug or loose they are in relation to the stock. Also where the sights are placed (and what they weigh) and where the barrel is breeched (and at what tension) and where the touch hole is located, and what kind of flint is used or what brand of percussion cap, and that's why, with a given rifle, if you change any of the nodes along the 'string' (in this case the barrel) you will make subtle changes on the rate of vibration and therefore in the downrange strike of the projectile.
Put without the technospeak:
save your bucks, man, use the extra money to buy more powder, and then go down to the range and burn the powder:
make good smoke!
(just one guy's opinion)