=Operation of the Breech Mechanism.= When the breech is closed and locked, the threaded portions of the breechblock mesh with the threads in the breech recess. The operating lever is held by the lower end of the operating lever handle which engages the block carrier lever catch, thus preventing any rotary motion of the breechblock at the instant of firing. The firing pin receives the blow of the percussion hammer and fires the primer. The flame passes through the vent in the obturator spindle, igniting the propelling charge. The gas pressure in the bore forces the mushroom head of the obturator spindle hard against the gas check pad causing the latter to expand and press against the walls of the chamber, forming a gas-tight joint. After the explosion the elasticity of the pad causes it to resume its former shape, allowing the obturator to be withdrawn freely from its seat when the breech is unlocked. =To Open the Breech.= After the piece has been fired, and before unlocking the breech, press back the firing mechanism block safety latch, screw out the firing mechanism block and remove the used primer. The breech can not be unlocked with the firing mechanism block in place. An attempt to do so will result in jamming of the firing mechanism safety plunger. It is therefore important that the firing mechanism block should be removed before attempting to unlock the breech. Press down on the handle of the operating lever in order to disengage it from the block carrier lever catch. Move the lever toward the rear and then to the right. In the first part of this movement, the operating lever turns freely around the hinge pin and its lug operates the rack which turns the breechblock. The threaded parts of the breechblock are thus disengaged from the threads in the breech recess. As the rack reaches the limit of its travel, the block carrier is swung on its hinge drawing the breechblock out of the breech recess. As the block carrier leaves the breech face of the howitzer the rack lock is forced by its spring into the recess in the rack preventing any further rotary motion of the breechblock in either direction. As the breech reaches its full open position the right end of the operating lever catch engages the operating lever catch, locking the breech in open position. In loading, care should be taken to ram the projectile home and to enter the propelling charge in such a way that the igniter of the base charge will be in contact with the mushroom head of the obturator spindle when the breech is closed.