looking into the barrel of a 20mm antimateriel rifle. The grooved surface of the barrel helps to sping the bullet, making it fly in a straighter line. <p>This particular rifle barrel will fire a 100 gram projectile at 1050 meters per second or so, and so is capable of putting a football sized (220 mm) hole through a concrete wall, or penetrating a 13 mm (½ inch) thick steel plate.<p> (btw this barrel was not attached to the rifle when this picture was taken; do not point firearms/artillery at your face if you can help it)