The Breech mechanism closes and seals the gun barrel's powder chamber after the projectile and powder charge are loaded. The breechblock, the principal part of the breech mechanism, is either a sliding-block or a threaded plug design. The breechblock must open and close quickly while making a high-pressure gas-tight seal. A typical gas pressure approaching 50,000 psi is found in the chamber and gun barrel shortly after the powder charge is ignited.