If your barrel is long enough to get a bullet spinning at the barrel's twist rate, making it longer will not improve accuracy. For example, in a handgun, 2.5" is a long enough barrel for this. It MAY APPEAR that longer barrels are more accurate, if you are using iron sights, because the distance between sights (sight radius) is longer, permitting more precise aiming. With a scope sight, this advantage disappears. As a matter of fact, often shorter barrels are stiffer, and consequently MORE ACCURATE than longer whippy ones.

However, the longer the barrel, the greater the length of time the powder gases have to push on the bullet, and consequently, the higher the bullet's velocity will be.[8D]