The burn time of powder has nothing at all to do with the length of the barrel of the gun, but rather the amount and type of powder in the case.(ball, flake, etc) Ideally, the "all burnt" point of the propellant is achieved prior to the bullet exiting the muzzle. In short barreled weapons, any unburnt powder still remaining after the bullet exits results in excessive muzzle flash. A 6 inch barrel offers greater accuracy due to the extra rifling inside, causing more spin to the projectile. If recoil is a concern (it shouldn't be with anything under a .357) consider getting the barrel ported.