A firearm such as a rifle, pistol, or artillery piece fires a projectile as a result of the burning of its propellant, which usually is smokeless powder. When the powder is ignited, large quantities of gases are produced. These gases force the projectile through the barrel in much the same way that children blow peas from a pea-shooter. If the gas pressure is too small, the bullet or shell will not reach its target. If the pressure is too great the gun may blow up. Also, if the pressure changes with each shot, the velocity of the projectile will change and accuracy will be poor. How to regulate this pressure is one of the most important problems in interior ballistics.