The optimum barrel length is something of a moving target :-) To the
first approximation the longer the barrel The faster the bullet. Adding
the friction factor greatly reduces this number from infinity to several
inches. The type and age of the powder used, the mass and composition of
the bullet, and the mechanism of the firearm, all play a factor in
determining the optimum barrel length. Firearm enthusiasts spend a great
deal of time determining the optimum load for a given firearm. It is much
easier to change powder and projectile than barrel length in most cases.