the slide (see the Gun Assembly picture above). The hydraulically driven rammer spade, called the power spade in that picture, is at the back of the rammer tray. If the multiple names of the "spade" are confusing, look at this footnote. Hand ramming was also possible in case of power failures with rate of fire approximately cut in half. Vertical sliding-wedge breech block (see drawing) The breech block closes the chamber behind the propellant case. It also contains the firing pin assembly. Hydraulic recoil Two hydraulic pistons in the housing absorb the major shock of recoil as the housing moves back inside the slide (see picture). They also buffer the end of counter-recoil for a soft return to battery. Pneumatic counter-recoil At the end of recoil, the counter-recoil system moves the housing forward again until it is back "in battery," and holds it there at any gun elevation. A chamber in the housing is filled with compressed air. At the rear of this chamber is a cylindrical hole with a chevron packing. Projecting through the hole is a movable piston that abuts the back of the slide (see picture). The air pressure in the chamber attempts to push the piston out of the chamber, but the piston can't move because of the slide. Therefore, since the piston can't go backwards, the air pressure in the chamber forces the housing forward. When the gun is in battery, the pressure in this chamber is about . During recoil, the pressure rises to about . Barrel designation (5"/38 Caliber) 5" is the caliber of this gun. The term "caliber" used here is the artillery caliber definition, and its value is equal to the bore diameter as measured from land to land. Lands are the raised portions of the bore rifling, between the grooves. (note: artillery caliber and small arms caliber are not the same). 38 caliber means that the barrel from breech face to muzzle is 38 calibers in length. As this gun's caliber is 5 inches (127mm), its barrel length is 38 times 5 inches: . Barrel description Radially expanded monobloc steel alloy. Weight: The bore is chrome plated from the origin of rifling (forward end of the chamber) to the muzzle. 45 groove rifling with a full uniform right hand twist in 30 Calibers (). Maximum bore pressure of a new gun firing an anti-aircraft common projectile: . Barrel connected to