Abstract: A gun barrel adapted to be connected to a receiver includes a rifled portion and a smoothbore portion. The rifled portion may have deeper than normal grooves to permit the escape of propellant gases past the bullet. The smoothbore portion includes an increased diameter expansion section, a reduced diameter compression section and an alignment section. Gases expanding past the bullet reduce the peak pressure in the gun barrel and provide a relatively low pressure adjacent the muzzle at the time of bullet exit. The improved gun barrel affords increased bullet velocity and accuracy, and reduced felt recoil.