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: This opens and closes the breech.
 * Breech Operating Mechanism: This actuates the breech block.
 * Firing Mechanism: This fires the round.
 * Obturator: In weapons using separate-loading ammunition, this provides a gas seal.

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.
 * 1. The threaded surface of the breech block is divided into an even number of sections, and the threads of the alternate sections are cut away (fig 4-14). As a result, half of the breech block surface is threaded and half is plain or slotted. Similarly, the threads of the breech recess are cut away opposite the threaded sections of the breech block. The breech block can be rapidly inserted into the breech recess, with the threaded sections of the breech block sliding in the blank sections of the breech recess. With a small turn, the threads of the breech block and breech recess are fully engaged and the breech block is locked. If the threads were not cut away, locking the breech block would require several turns. This type of interrupted-screw breech block is used on the M109/M109A1 howitzer and the M198 howitzer.


 * 2. The stepped-thread interrupted-screw (Welin) breech block (fig 4-15) is a modification of the interrupted-screw breech block. The breech recess and breech block are cut with a series of stepped threads. When the breech block is inserted and turned in the breech recess, the matching sections of stepped threads engage. Less rotation is needed to close the breech block, and there is a greater threaded surface or holding area. The stepped-thread interrupted-screw breech block is used on the M114/M114A1/M114A4 howitzer, the M110/M110A1/M110A2 howitzer, and the M107 gun.
 * Figure415 Stepped-thread interrupted-screw breech block.png

Sliding-Wedge

 * 1. Description. The rectangular sliding-wedge breechblock slides in a rectangular recess in the breech ring. It is used on cannons firing semi-fixed ammunition.


 * Figure418 Sliding-wedge breechblock operation.png
 * (a) When the motion of the breech block is horizontal, the mechanism is referred to as the horizontal sliding wedge breech block (fig 4-16). The M101/M1OlA1 howitzer has this type of breechblock.


 * (b) When the motion of the breechblock is vertical, the mechanism is referred to as a vertical sliding-wedge breechblock (fig 4-17). The M102 howitzer has this type of breech block.


 * 2. Operation. Figure 4-18 shows the operation of the sliding-wedge breechblock.


 * (a) In view A, the breechblock is open, and the cannon is ready to receive the ammunition.


 * (b) The round is pushed into the breech almost to a seated position as shown in view B.


 * (c) With the action of the operating mechanism, the breechblock slides over the breech face (view C). The beveled edge of the breechblock forces the round into its seated position in the chamber. The wedging action between the breech-block and the breech ring seals the tube.

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.


 * (a) To open the breech, the safety latch is released and the operating handle is pulled down to a horizontal position. This rotates and unlocks the breechblock. The operating handle is then used to swing the carrier and breechblock open. To close, the operating handle is used to swing the carrier and breechblock to the closed position. Raising the operating handle to the upright position rotates the breechblock to the locked position.


 * (b) A counterbalance mechanism (fig 4-20) is provided on some weapons to aid in swinging the breechblock into the open and closed position with ease, safety, and rapidity. Although useful at low elevation to hold the breechblock in the fully open or closed position, the counterbalance mechanism is especially valuable when the cannon is elevated, because it aids in swinging the breechblock against gravity and lowering it gently. The counterbalance mechanism is essentially a cylinder, piston, piston rod, and compression spring connected to the breech mechanism. The compression spring pulls and holds the breech-block in the open or closed position.

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.


 * (a) When the operating handle is unlatched and rotated to the rear, it cams the breechblock horizontally to the right, thereby opening the breech (view A, fig 4- 23).


 * (b) After a round is inserted in the chamber, the breechblock is cammed back into the breech ring when the operating handle is manually rotated forward (view B).The front face of the breechblock is beveled and, as the breech is closed, the beveled face contacts the base of the cartridge case and seats it in the chamber. When the breech is completely closed, the operating handle engages and is locked in the closed position by a catch located in the upper right corner of the breech ring.


 * (c) The extractor (fig 4-24) is seated in the bottom of the breech ring under the breech-block. As the breech is closed, the camming groove in the breechblock rotates the extractor about its seat in the breech ring until the lip end of the extractor is in a recess in front of the rim of the cartridge case (view A, fig 4- 25).When the breech is opened, the extractor lip, which is engaged in the rim of the cartridge case, is cammed sharply to the rear, thereby extracting the cartridge case from the chamber (view B).

(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.


 * (a) When the operating handle is unlatched and rotated to the rear, the breechblock drops, opening the breech (view A, fig 4-26).When the breechblock drops, it moves the extractors to the rear, extracting the fired cartridge case, and loads the coil spring in the operating shaft. The downward motion of the breechblock also cocks the percussion mechanism. The extractors remain in the rearward position to lock the breechblock in the open position.


 * (b) When a round is inserted in the chamber, the extractors are pushed forward, unlocking the breechblock. The operating handIe is rotated forward to close the breech (view B). When the breechblock is completely closed, the operating handle engages and is locked in the closed position by a catch in the operating handle stop.

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:
 * (1) Cocking phase (view A, fig 4-31).


 * The cocking lever is rotated. The lower end of the lever pushes against a lug in the guide, moving the guide rearward and compressing the firing spring until the sear lug passes the sear. The action of the sear spring moves the sear across the guide in front of the sear lug. The fully compressed firing spring pushes the guide and firing pin forward. The sear lug pushes against the sear, which prevents the guide from flying forward. The weapon is now ready for firing.


 * (2) Firing phase (view B).


 * When the lanyard is pulled, the firing trigger is rotated, which in turn causes the sear to revolve. The rotation of the sear causes the engaging surfaces to tip over, releasing the inertia mechanism. The sear releases the guide, which moves forward under the pressure of the firing spring. The firing spring stop strikes the forward end of the breechblock, inertia carrying the guide forward until it strikes the breechblock; at the same time, the firing pin hits the primer and fires the weapon. The forward motion of the firing pin reduces the distance between the shoulder on the firing pin and the firing spring stop, thereby compressing the retracting spring between them.


 * (3) Retracting phase (view C).


 * The retracting pin pulls back or retracts the firing pin into the breechblock to keep the firing pin from being sheared off as the breechblock is opened. The compressed retracting spring pushes back on the firing pin and is restrained only at its forward end by the firing spring stop and breechblock. The guide, with firing pin attached, is free to move to the rear. The compressed retracting spring expands, pulling the firing pin back into the breechblock. The system is then at rest, ready for subsequent cocking and firing.

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:


 * (1) A primer is inserted in the primer holder in the front face of the firing mechanism block (fig 4-32).


 * (2) The firing mechanism block is screwed into the breechblock (view A, fig 4-33).


 * (3) To fire, the lanyard is pulled (view B). When the lanyard is pulled, the hammer strikes the firing pin, which then strikes the primer.


 * (4) The compressed firing pin spring returns the firing pin to its normal position (fig 4-32).


 * (5) After firing, the firing mechanism block is unscrewed from the breechblock to remove the fired primer.

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.


 * (1) In weapons using semifixed ammunition, obturation is accomplished by means of the cartridge case. When the round is fired, the pressure of the propellant gases expands the cartridge case, forming a tight seal against the walls of the chamber. This prevents the gases from escaping to the rear. When the gas pressure drops, the cartridge case contracts sufficiently to permit easy extraction.


 * (2) In weapons using separate-loading ammunition, an obturating device, such as the DeBange obturator, must be included in the breech mechanism to prevent the rearward escape of propellant gases.

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:
 * Bag Ammunition - Ammunition in which fabric bags are used to hold the propellant (gun cotton, cordite, smokeless powder among others) and the projectile is handled separately. Propellant bags were primarily manufactured from a raw silk also known as "cartridge cloth" or else from a special coarse wool twilled on both sides known as "shalloon." Unlike cotton, these materials burn without leaving any smoldering residue in the barrel which would present a safety hazard when loading the subsequent round. Shalloon was used by most nations as it was relatively inexpensive but silk was preferred in the USN as it reduced barrel wear. Bags made from Rayon rather than silk were used in the US Navy for some guns after a serious propellant fire aboard USS South Dakota (BB-57) in 1945 was traced to a spark generated when a silk bag was removed from its metallic container.
 * Bag Gun - Weapons that used powder bags rather than a cartridge case to hold the propellant. This was the most common ammunition type of the twentieth century for 6 inch (15.2 cm) and larger guns. Guns using powder bags were designated as "Separate Loading" in the US Navy, a reference to the normal procedure for bag guns of first ramming the projectile and then the powder bags. Smaller calibers generally use a single bag while larger calibers may have as many as six bags.
 * Breech - The rear of the gun. In most 20th century guns, where the shell goes in, hence the name "breech-loading weapon."
 * Breech, Four-Motion - Early screw breech design invented by the French where all of the threads were at the same level. Usually there were five or six threaded sections separated by equal width non-threaded sections. These breechblocks were comparatively long as half of the thread was cut away to allow withdrawal, thus reducing their capacity to withstand firing stresses. This type of breech mechanism required that the block be first withdrawn straight back from the gun barrel before it could be moved out of the way of the reloading process and some had to be disconnected entirely from the gun. Closing the breech was a complicated operation, involving four motions: swing the carrier so that the screw could enter the breech, slide the entire breech mechanism forward so as to seat it, turn the screw through part of a turn until the threads meshed with those in the breech, and then lock the mechanism.
 * Breech, Holmstrom - A type of breech mechanism for bag guns whereby the screw block was operated by a crank that moved parallel to the rear face of the block. Named after the inventor, Carl Holmström.
 * Breech, Pure-Couple - British Welin Breeches of the early 1900s that used a long, manually operated lever to operate the breech screw. A "couple" consists of two parallel forces that are opposite in direction and do not share a line of action. A couple does not produce any translational movement, only rotation. A "pure couple" always consists of two forces that are equal in magnitude. So, a "pure couple breech mechanism" is one that uses and produces rotational force in order to work the breech plug. See "Breech, Welin" below.
 * Breech, Single-Motion - See "Breech Mechanism, Smith-Asbury" below.
 * Breech, Three-Motion - Similar to a Four-Motion Breech, except that the breech was withdrawn into a hinged carrier ring. This meant that the motions were reduced to unlocking, withdrawal and then swinging out of the way.
 * Breech, Welin - A stepped-thread breech developed during the 1890s and named after the inventor, Axel Welin, a Swedish Ammunition Engineer then residing in London. A Welin breechblock has only one third or one quarter of the threads removed, which means that a shorter screw length can be used to obtain the same strength as a four-motion breech mechanism. See the "breech plug" in the illustration above for a typical Welin design. Unlike the four-motion breech, a Welin breech allows the mechanism to be simply unscrewed and swung out of the way, it does not have to be withdrawn straight back from the gun barrel. Variations of the Welin Breech design were used on most bag ammunition guns of the twentieth century.
 * Breechblock, Vertical Sliding-Wedge - Used on many cartridge guns, this sort of breech mechanism slides vertically in grooves cut in the housing. After the cartridge is inserted, the breechblock slides up with the sloping part of the forward face wedging the cartridge home in the chamber. An extractor mechanism catches the cartridge case rim as the breechblock drops after firing, pulling the expended cartridge case out of the gun. A few guns use a similar design, but with the breechblock sliding horizontally. These, of course, are known as "Horizontally Sliding-Wedge" breechblocks.
 * Breech Mechanism, Smith-Asbury - Introduced in 1916 and named after its inventors, Lt. Cmdr. George Leonard Smith, USN and Draftsman Dorsey Frost Asbury, both of the Naval Gun Factory at the Washington Navy Yard. This mechanism used gearing to fully open a Welin breech by linking the unscrewing, withdrawing and swinging clear movements into one continuous action and for that reason is sometimes known as a "single motion" breech mechanism. The breech threads are undercut at the start so as to allow the block to swing into position. For details and drawings see "Asbury Breech Mechanism" Patents 1020849 (GL Smith and DF Asbury), 1323947 (DF Asbury) and 1323947 (GL Smith).
 * Driving Band or Rotating Band - A raised ring or rings of soft metal encircling a projectile designed for rifled gun barrels. Bands are normally located near the base of the projectile. The bands engage the rifling in the gun barrel, causing the projectile to spin as it travels through the barrel. Additionally, they provide a tight seal so that the propellant gases do not escape past the projectile, help to center the rear end of the projectile in the bore and hold the projectile in place during loading and gun elevation. Bands are typically made of copper, brass or soft steel. USN driving bands on large caliber projectiles were an alloy consisting of 97.5% copper and 2.5% nickel. An "augmented driving band" or "augmented rotating band" is a slightly thicker strip of metal used when the rifling in the gun barrel has been worn down to the point that a standard driving band is no longer effective.
 * Mushroom head - A component of Welin breechblocks, this is the forward part of the breechblock, between the chamber and the obturator. See the diagrams for the "Breechblock" and "Obturator."
 * Obturator - For breech loading guns, this is a device for making the breech gas-tight, preventing any escape of propellant gas while the breech is closed. For bag guns using Welin breechblocks, the De Bange obturator system (named for Captain Charles Ragon de Bange of the French Army, who invented it in 1872) was commonly used. The De Bange obturator system consists of a doughnut-shaped washer pad, also known as a "gas check pad," that is located between the mushroom head and the screw breech. See the breech illustrations for details. When the breech is closed, rotating the screw squeezes the pad against a conical section of the gun barrel. When the gun is fired, the mushroom head is driven back against the pad which forces it tighter into the conical seating and thus seals the barrel. For guns using cartridge ammunition, the cartridge case itself becomes part of the obturator system. Generally speaking, both the cartridge case lip and cartridge case mouth are used to seal the barrel. The cartridge case lip is forced tightly against the breech when closed while the chamber of the gun is designed such that the expansion of the mouth of the cartridge case when the gun fires helps to seal the barrel.
 * Primer Vent or Vent Axial - A feature of the breech mechanism used for bag guns, this is a hollow tube running from the firing lock or primer chamber through the stem to the front of the mushroom head. When the igniter or primer is fired, it generates a flame which travels through this tube into the propellant charges.
 * Rifling - The bores of most gun barrels have grooves milled into them in a spiral pattern. These grooves engage the Driving Bands on the projectiles and thus impart a spin to them as they leave the barrel. Spinning the projectiles makes them more stable in flight which greatly increases their accuracy and range. It also makes them more likely to land nose first, which is very important for an armor-piercing shell. The way the grooves are milled varies greatly. Some manufacturers prefer a uniform pattern, others prefer grooves that vary in depth and width as they progress through the weapon.