Breech mechanism 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 . Weapons smaller than 155mm are usually of one of several sliding - block designs . Larger weapons are usually of one of several interrupted - screw designs . For rapid - fire guns , a sliding breech - block is commonly used which may open and close automatically as the gun is fired . Recent large weapons have incorporated automatic or semi - automatic breechblocks , more than doubling the weapon ' s rate - of - fire . Using Time On Target methods such as " Multiple Round Simultaneous Impact ( MRSI ) " , one modern 155mm gun can fire eight projectiles , with all eight landing on the same target within four seconds , a task that normally requires firing eight individual guns at the correct calculated times . General description . The breech mechanism is the group of devices that opens and closes the breech , fires the round of ammunition , and provides a gas seal after firing . The breech mechanism ( fig 4 - 12 and 4 - 13 ) consists of the following : Breechblock types . The breech block is the principal part of the breech mechanism . It is essentially a large heavy piece of steel that covers or seals the breech end of the tube and houses portions of the firing mechanism . There are two types of breech - block - the interrupted screw and sliding - wedge . Interrupted - Screw . This type of breech block is used on cannons that fire separate - loading ammunition . Breech Operating Mechanism . The breech operating mechanism is primarily designed to actuate the breechblock . The carrier and sliding - wedge are the two main types of breech operating mechanisms . Carrier Breech Operating Mechanism . The carrier breech operating mechanism is used in cannons with interrupted - screw breechblocks . The carrier , which is assembled to the breechblock , is hinged at the side so that it swings the breechblock horizontally . The carrier has a mechanism for rotating the breechblock to disengage the threads before swinging it free of the breech and for engaging the threads after closing . The carrier breech operating mechanism is designed for manual or semiautomatic operation . Manual operation . The operating handle on this type of breech operating mechanism ( fig 4 - 19 ) actuates the rotating mechanism of the breechblock carrier and swings the breechblock out of its recess in the breech ring . A counterbalance mechanism aids in opening and closing the breech on some medium - and large - caliber weapons that have large and correspondingly heavy breech - blocks and carriers . Semiautomatic operation . The rotation of the operating crank on this type of breech operating mechanism ( fig 4 - 21 ) actuates the rotating mechanism of the breechblock carrier and swings the breechblock out of its recess in the breech ring . To load the first round , the breech must be opened manually . After firing each round , the breech is automatically opened during counterrecoil . Manual operation . In figure 4 - 22 ( a ) , the breechblock is opened by depressing the plunger on the operating handle and pulling the operating handle rearward until the operating crank pin is engaged in the cradle cam . The operating handle is returned to the locked position . Automatic operation . In figure 4 - 22 ( b ) , during counterrecoil , the operating crank pins enter the cam grooves on the underside of the cradle cam . As the cannon moves into battery , the cam grooves , acting on the operating crank pins , force the operating crank to rotate . This actuates the rotating mechanism of the breechblock carrier and causes the breechblock to rotate , unlock , and swing open . Closing the breech . In figure 4 - 22 ( c ) , when the breechblock is held in the open position with the operating crank pin engaged in the cradle cam , the closing springs inside the operating crank are under tension . To close the breech , the cradle cam is lifted up until the operating crank pin is disengaged from the groove in the cradle cam . The tension of the closing springs rotates the operating crank , causing the breechblock carrier to swing the breechblock closed , rotate , and lock . The breech operating mechanisms for horizontal and vertical sliding - wedge breechblocks are manually operated and are similar in operation . The motion of the breechblock is used to activate the mechanism for extracting the cartridge case after firing . ( 1 ) Horizontal sliding - wedge breech operating mechanism . The horizontal sliding - wedge breech operating mechanism is operated by means of the operating handle . ( 2 ) Vertical sliding - wedge breech operating mechanism ( fig 4 - 26 ) . The vertical sliding - wedge breech operating mechanism is operated by the operating handle . A coil spring in the breechblock operating shaft functions as an equilibrator to equalize the effort required to open and close the breechblock . Firing mechanism types . The firing mechanism is a device for initiating the ammunition primer . The primer then sets off the igniter which ignites the propelling charge of the ammunition . Current field artillery weapons have a percussion firing mechanism activated by a lanyard . The percussion firing mechanism is also characterized by a firing pin which strikes the primer of a round . The three types of percussion firing mechanisms are continuous - pull , inertia , and percussion - hammer . Continuous - Pull . One continuous pull of the lanyard initiates the complete functioning of the firing mechanism . This safety feature ensures that the firing mechanism is not cocked except at the instant before firing . The M13 and M35 firing mechanisms are the two continuous - pull firing mechanisms in use . M13 . Referred to as the firing lock , this firing mechanism is used on the M101A1 howitzer and functions as follows : ( a ) Pulling the lanyard causes the trigger shaft to rotate , which forces the trigger fork forward ( view A , fig 4 - 28 , sheet 1 ) . This pushes the firing pin holder sleeve forward , compressing the firing spring . ( b ) The sleeve continues to be forced forward until it trips the sear ( view B ) . ( c ) This releases the firing pin holder , allowing the compressed firing spring to expand and snap the firing pin forward ( view C , sheet 2 ) . The firing pin detonates the primer in the cartridge case , which ignites the propelling charge . ( d ) When the pressure on the trigger fork is released , the firing spring continues to expand forward and rearward with equal force . The forward pressure is applied to the middle rear surface of the trigger fork by the T on the firing pin holder ( view D ) . The rearward pressure is applied to the top front surface of the trigger fork by the firing pin holder sleeve . ( e ) Since the rearward pressure exerted to the top front surface acts on a longer lever arm than that of the forward pressure , the trigger fork rotates rearward and moves the firing pin holder rearward until the sear again engages the firing pin holder ( view A , sheet 1 ) . M35 . In figure 4 - 29 , the functioning of this firing mechanism is very similar to the M13 firing mechanism . It is used on the M109 / M109A1 howitzer , the M110 / M11OA1 howitzer , the M198 howitzer , and the M107 gun . Inertia . In Figure 4 - 30 , this firing mechanism has a heavy firing pin and guide assembly which moves forward by inertia to strike the primer after the action of the firing pin has stopped . The firing pin and guide assembly is retracted by a separate retracting spring . The firing mechanism cocks when the breechblock is opened and remains cocked during loading and closing of the breechblock . The inertia firing mechanism , used on the M102 howitzer , functions in three phases : Percussion - Hammer . In Figure 4 - 32 , this firing mechanism has a hammer which , when actuated by a pull on a lanyard , strikes the firing pin and fires the weapon . The percussion - hammer firing mechanism , used on the M114A1 howitzer , functions as follows : Obturator . Obturation is the prevention of the rearward passage of gases into the threads and other parts of the breech mechanism . These gases , which have great velocities and high temperatures , would soon erode and ruin the breech mechanism . This would affect the ballistics of the weapon if a means of obturation were not introduced . DeBange Obturator . Components . The obturator , which passes through the breechblock ( fig 4 - 34 ) , is free to move back and forth . Between the mushroom - shaped head of the spindle and the breechblock is the obturator pad ( fig 4 - 35 ) made of asbestos and nonfluid oil or of neoprene . Two steel split rings , ground accurately to bear against the walls of the bore , encircle the obturator pad . The obturator pad , split rings , and spindle do not rotate with the breechblock during closing or opening , but the breechblock rotates about the spindle as an axle . There is a small split ring around the spindle to prevent the escape of obturator pad composition and gas . A disk forms a bearing between the obturator pad and breechblock . Operation . When the weapon is fired , the gas pressure acts against the head of the spindle , moving the spindle back and compressing the pad between the rear of the mushroom head and the front of the breechblock itself ( view A , fig 4 - 36 ) . This causes the pad to expand radially against the split rings , which expand to make a gas - tight seal against the bore wall . After firing , when the gas pressure is reduced , the pad returns to its normal shape ( view B ) , moving the spindle forward . The split rings contract to their original size , and the breechblock can then be opened . Glossary . The following terms are used in the component and operation of breech mechanisms :