40 MM Antiaircraft Gun, OP 820, 1943, is a Navy service manual for the most widely used anti-aircraft gun of WW II.
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NAVY DEPARTMENT BUREAU OF ORDNANCE WASHINGTON, D. C.
ORDNANCE PAMPHLET No. 820 40MM ANTIAIRCRAFT GUN
1. Ordnance Pamphlet No. 820 provides a description of the construction and operation of the 40MM Machine Gun Mechanism, and descriptions of the gun barrel and sight. It also includes operating instructions and instructions for the care and maintenance of the gun.
2. Ordnance Pamphlet No. 820-A may be used where a less detailed pamphlet is desired.
3. This pamphlet supersedes Ordnance Data No. 3781, preliminary manual for the description and operation of the 40MM antiaircraft gun. Ordnance Data No. 3781 should be destroyed. Ballistic Data for 40MM Guns are given in Ordnance Pamphlet No. 867. This pamphlet supersedes OP. 820A, which should be destroyed.
4. Ordnance Pamphlet No. 820 is UNCLASSIFIED.
Rear Admiral, U.S.N.
Chief of Bureau of Ordnance
This Ordnance Pamphlet, No. 820, applies
to the following:
40MM MACHINE GUN MECHANISMS, MARK 1 AND MARK 2, MARK 1, MOD. 1 AND MARK 2, MOD. 1
40MM GUN BARREL, MARK 1
40MM SIGHTS, MARK 3 AND MARK 4
Standard Navy nomenclature is used
herein, and is considered preferable to that
used on drawings and in previous publications relating to these mechanisms.
The 40MM Antiaircraft Gun, Figure 1, consists of the machine gun mechanism, the gun barrel, and the sights. The 40MM Machine Gun Mechanisms are assembled in pairs. The left mechanism is designated Mark 1 or Mark 1, Mod. 1, and the right mechanism is designated Mark 2 or Mark 2, Mod. 1. The only differences between the two Marks of mechanisms are those required by the twin assembly. The only difference between the Mod. 0 and Mod. 1 is that certain parts are not interchangeable between the two. 40MM Gun Barrels, Mark 1, are used with all mechanisms. The 40MM Sight, Mark 3, is used on the 40MM Twin Mount; the 40MM Sight, Mark 4, is used on the 40MM Quadruple Mount.
The design of these mechanisms and barrels is essentially that of the Swedish Bofors 40MM Antiaircraft Gun. The design provides for a rapid fire, recoil operated, automatic mechanism, with a maximum cyclic rate of approximately 160 rounds per minute.
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Part One-Description of Gun Mechanism
The 40MM Machine Gun Mechanism consists of five principal components. They are the Slide Assembly, Breech Mechanism Assembly, Loader Assembly, Recoil Cylinder Assembly, and Barrel Assembly (excluding barrel).
A. SLIDE ASSEMBLY
The slide, Figure 2, is a steel casing which positions and provides working surfaces for other parts of the mechanism. It has a trunnion, trigger
mechanism, hand operating mechanism, top, side, and rear doors, bottom cover and extractor release lever. A Mark 1 and a Mark 2 Mechanism are bolted together and are supported at the trunnions, one on the left side of the Mark 1 Mechanism, and one on the right side of the Mark 2 Mechanism. An elevating arc is attached to the bottom surfaces of the slides.
The trunnion on the left is part of the Mark I Mechanism, the one on the right is part of the Mark 2 Mechanism.
The trunnion, Figure 3, contains the firing plunger and crank of the trigger mechanism. The trunnion of the Mark 1 Mechanism is provided with a gear sector to transmit gun elevation to the firing cut-out mechanism of the mount.
2. Trigger Mechanism
The trigger mechanism, Figure 4, is on the left side of the Mark 1 Mechanism on the right side of the Mark 2 Mechanism. It is operated by the firing plunger in the trunnion. The trigger is a vertical arm, pivoted near its center, and is located inside and at the rear of the slide, where it contacts and controls the movement of the rammer control spindle arm of the loader. The trigger is operated by a pawl on the firing lever. Motion of the trigger and firing mechanism is limited by the cam on the firing selector lever shaft, as shown in Figure 5. When the firing selector lever is at SAFE, motion of the trigger mechanism is prevented by the firing selector cam. When the selector lever is at AUTO FIRE, the trigger can be maintained in the firing
position by the trigger mechanism. When the selector lever is at SINGLE FIRE, motion of the trigger mechanism can be sufficient to permit the pawl to trip and thus release the trigger. The trigger returns to its non-firing position after releasing the trigger catch lever in the loader. Thus in SINGLE FIRE the firing mechanism must be restored to its non-firing position before the trigger can again be operated.
Phantom positions of the rammer control spindle arm show how it contacts the trigger when the loader is placed in the slide. The position of the hand operating lever shaft is shown when the hand operating lever is in the rear catch bracket.
3. Hand Operating Mechanism
The hand operating mechanism, located on the same side of the slide as the trigger mechanism, provides the means for performing manually the operations required to prepare the gun for firing, operations which are otherwise accomplished automatically during recoil and counterrecoil. It
FIRING SELECTOR LEVER POSITIONS
consists of the external hand operating lever and associated internal linkage. The arm on the hand operating lever shaft, Figure 4, operates the cocking mechanism of the loader. A projecting lever of the shaft is pinned to the hand operating rod, which engages the toe of the breech mechanism outer crank to cock the breech mechanism. The lever also provides an interlock with the trigger mechanism to prevent firing when the hand operating lever is in the rear catch bracket.
4. Top Door
The top door, Figure 6, when open, disengages the barrel lock, Figure 7, and rotates the safety catch arm of the housing into position to engage a stop on the slide. This door must be left latched open when the barrel assembly is not in place, to prevent possible rearward movement of the housing.
However, the top door must always be latched closed before firing, in order to disengage the safety catch arm from the stop, thus preventing damage to the mechanism.
The top door, when open, disengages the barrel lock.
The barrel lock is shown in both open and closed positions.
SIDE DOOR CAM ACTION
This transparent view shows the action of the cam surface of the side door on the outer crank. The crank is shown at the start and finish of its travel during recoil.
5. Side Door
The side door has a cam surface shown in Figure 8, to provide for the rotation of the outer crank of the breech mechanism during recoil.
This door must be opened to remove the breech block, closing spring, and cranks from the housing. However, the side door must always be locked closed before firing, so that the breech block will be lowered during recoil. Otherwise the breech block will be rammed into the front of the loader causing serious damage.
6. Rear Door
The rear door, Figure 9, has an opening to permit the recoil of the rammer tray and ejection of empty cases, and to facilitate removal of live rounds from the tray. It opens to permit removal of the loader and housing assemblies.
To this door are attached case deflector brackets on which are mounted the case deflector, counterweights, and recoil indicator. This indicator measures the length of recoil, being contacted by the rear of the tray in recoil. It must be reset by hand.
7. Bottom Cover
The bottom cover provides access to the breech mechanism and permits removal of the breech block and associated parts.
8. Extractor Release Lever
The extractor release lever, located in the bottom of the slide, provides for manually releasing the extractors from the breech block.
B. BREECH MECHANISM ASSEMBLY
The breech mechanism assembly consists of a housing assembly, breech block assembly and associated operating parts, and is a recoiling part of the gun mechanism.
1. Housing Assembly
The housing, Figures 10, 11, and 12, is a rectangular steel block with projecting bronze-covered bearing strips which ride in corresponding guides in the slide. The housing is provided with an interrupted thread for attaching the breech end of the barrel. The barrel lock is pivoted in the top of the housing
Broken open housing shows the relationship of the parts of this assembly.
This view of the housing shows the breech block in the closed position.
and locks the barrel in the assembled position when the top door is closed. The safety catch arm prevents rearward movement of the housing, when the top door is latched open, by contact against the stop fastened to the slide. A vertical slot is provided for the movement of the breech block. A transverse splined crankshaft keys together the outer crank, two inner
This is the complete assembly for Mark I Mechanism, as it appears from the rear outboard side.
cranks, and the cover of the breech block closing spring. These are shown in Figure 13. Cams on the inner cranks operate in slots in the sides of the breech block to open and to close and lock the block in the closed position. Additional cams on the inner cranks operate the outer cocking lever and the sear of the breech block. Extractors, operated by the downward stroke of the
breech block, extract the empty case from the barrel chamber, and lock the block in the open position against the action of the closing spring. The locking is accomplished by the hooks on the extractors engaging abutments on the breech block. The extractors are tripped by a live round catapulted into the chamber, or by manual operation of the extractor release lever. A spring loaded safety plunger, Figure 14, moves longitudinally in the housing and contacts a cam on the breech end of the barrel. This plunger moves forward
The safety plunger is shown held back by the cam surface on the breech end of the barrel.
when the barrel is removed, enters a notch in the breech block, and locks the block in the open position. The plunger may be retracted to release the block by the use of a special tool inserted in the top of the housing.
2. Breech Block Assembly
The breech block, Figure 15, contains the firing pin and spring, inner and outer cocking levers, and the sear and sear spring. The inner cocking lever holds the firing pin in the cocked position or releases it. The outer cocking lever, which is splined to the inner cocking lever, is acted upon by the cam of the left inner crank to cock the firing pin. In the cocked position, the sear is forced outward by its spring, locking the inner cocking lever. In the firing position, the sear is forced inward by the right inner crank, releasing the inner cocking lever and the firing pin.
The disassembled breech block shows all parts of this assembly and their identifying piece numbers. The firing hole bushing is not found in all blocks, as some have the firing hole drilled through the solid face of the block.
C. LOADER ASSEMBLY
The loader assembly, Figure 16, consists of feeding, ramming and cocking mechanisms and their associated parts. It is located in the rear of the slide, resting on guides for easy removal, and is locked in place by the rear door.
1. Feeding Mechanism
Live rounds are inserted in the loader in clips of four. The clips are automatically removed by the rear guide and ejected through the clip chute. The side frames include guides which contain the feed pawls and stop pawls, Figure 17, for automatically feeding single rounds onto the tray, during the counterrecoil stroke. The feed pawls are attached to a movable holder, which is positioned in the feed rod by a spring loaded plunger. In case of overload, as from a jammed round, the plunger will trip and release the feed
pawl holder as the feed rod completes a stroke. The loader contains two star wheels which are pivoted at the forward ends and positioned at the rear ends by spring loaded plungers, Figure 18. The star wheels, Figure 20, are locked by catch mechanisms, which are tripped by pawls on the top face of the tray, or released by the hand operating mechanism. When a round is forced through the star wheels by the round above, the case spreads the star wheels sufficiently to pass through as it rotates the star wheels one-quarter turn.
Rear of loader; note transparent treatment of the crosspiece cover to show the star wheel plungers.
2. Rammer Tray
The loader contains the rammer tray, Figure 19, which is attached to the rear of the housing and recoils with it. The tray contains the rammer shoe, its levers, and the rammer spring. The rammer shoe, in the cocked position, is so located that the base of the live round, as it is forced through the star
Below the rammer tray assembly, shown at the top of the picture, are the parts of a disassembled tray.
wheels, will fall into the slots of the rammer levers. The tray supports the live round before the round is catapulted, and provides a guide for the ejected case. Guides in the sides of the tray operate the feed rods during recoil and counterrecoil. The pawls on the top face of the tray unlock the star wheel catch mechanisms during counterrecoil at the instant the feed pawls force a live round through the star wheels onto the tray.
3. Catch Levers
The rammer shoe is latched in the cocked position by one of three levers in the base of the loader. The three levers, Figure 20, are:
Center-Tray Catch Lever Inboard-Loader Catch Lever Outboard-Trigger Catch Lever
These levers are pivoted on a common shaft and are of varying length so that they release the rammer shoe in the following order: Tray catch lever, loader catch lever, trigger catch lever.
a. The tray catch lever is operated, through a rocker arm, by a cam on the bottom of the tray, and is in the releasing position whenever the tray is within about one inch of battery position.
b. The loader catch lever is maintained in the releasing position by a linkage in the rear guide, whenever the loader contains more than one round above the star wheels. Its purpose is to prevent the gun from firing the round on the tray, when there are fewer than two rounds above the star wheels to push another round onto the tray in counterrecoil. This lever is provided to prevent loss of automatic functioning of the mechanism, by latching the rammer shoe in the cocked position, if the loader is supplied with ammunition at a rate less than the rate of fire. This lever releases the rammer shoe automatically upon the insertion of additional clips of ammunition.
c. The trigger catch lever is controlled by the rammer control spindle, and the arm of the spindle is contacted and controlled by the trigger of the slide assembly. The trigger catch lever is placed in the releasing position whenever the trigger is in the firing position.
4. Cocking Mechanism
The loader contains the cocking mechanism which is operated by the hand operating mechanism of the slide. The cocking mechanism forces the rammer shoe to the rear and thus compresses the rammer spring. At the same time, the cocking mechanism rotates the catch mechanisms so that a round can be forced through the star wheels. Motion of the hand operating lever, beyond the position necessary to latch the rammer shoe, causes the rammer levers to move to the rear end of the slots in the tray. When the rammer levers are in this position, they are spread by the slots, so that a round can be inserted in or removed from the tray through the opening in the rear door.
Figure 20, Page 35.
Transparent view of loader.
Important loader mechanism parts are shown in their proper relationship.
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D. RECOIL CYLINDER ASSEMBLY
The recoil cylinder assembly, Figure 21, provides the necessary retarding force during the recoil and counterrecoil strokes to limit the length of recoil and to control the velocity of counterrecoil. The recoil cylinder is attached under the forepart of the slide, and the piston rod is attached to lugs on the housing.
The recoil cylinder shown in the illustration has been cut open to reveal the relationship of its parts.
1. Piston Rod
The piston rod is attached to the housing and moves in recoil with it. The forward end of the rod is enlarged to form a piston, and contains a throttling bushing which provides an orifice for liquid passing between it .and the throttling rod. The inside surface of the piston rod has two grooves, tapered at each end, which control the flow of liquid around the valve seat.
2. Throttling Rod
The throttling rod is attached to the body of the recoil cylinder at the front end. It varies in diameter throughout its length, tapering to its smallest diameter near the front end, thus varying the opening between the rod and the bushing. The rear end of the rod carries the check valve which permits flow of recoil fluid through it only during the recoil stroke.
3. Needle Valve
The needle valve fits into a valve seat inside the throttling rod. By varying the setting of this valve, the velocity of counterrecoil, and consequently, the rate of fire of the gun can be controlled.
4. Recoil Fluid
The fluid used in the recoil cylinder is specified by O.S. 1324. This mixture is approximately sixty percent glycerine and forty percent water, with a small percentage of corrosion inhibiting compound added.
E. BARREL ASSEMBLY
For description of barrel assembly see Chapter III.
F. DIFFERENCES BETWEEN MARK 1 AND MARK 2 MECHANISMS
Since the two mechanisms are bolted together so that they elevate and depress as a unit, certain differences between the Mark 1 and Mark 2 Mechanisms are necessary.
1. Slide Assembly
To permit the gun to be loaded, cocked and fired, each mechanism has its hand operating lever, firing selector lever, trigger mechanism, and firing plunger on the outboard side of the slide. To allow access to the breech mechanism assembly, the side doors are on the outboard side. The extractor release levers are also placed near the outboard side of each slide. The top doors differ in having their catches on the inboard side.
2. Breech Mechanism Assembly
The housings in the Mark 1 and Mark 2 Mechanisms differ in having their breech closing spring assemblies mounted on the inboard side, while the outer cranks, the extractor spindle arms, and safety catch arms are on the outboard side. The breech closing springs differ in the direction of their spirals; the closing spring in the Mark 1 Mechanism is wound so that it increases in diameter in a counter-clockwise direction, when viewed from the convex side of the spring; in the Mark 2 Mechanism, the spring is wound in the opposite direction.
Extractors and breech block assemblies are identical for the two Marks of mechanisms, and are, therefore, interchangeable.
3. Loader Assembly
The differences between the loaders in the Mark 1 and Mark 2 Mechanisms are those which are required to permit the cocking of the rammer and release of the catch mechanisms by the hand operating lever, the operation of the rammer control spindle arm by the trigger, and ejection of the ammunition clips through the outboard side of the slide.
4. Recoil Cylinder Assembly
The only difference between the recoil cylinders in the Mark 1 and Mark 2 Mechanisms is that the fill plug of each is positioned on the outboard side for easy access in filling the recoil cylinders.
Figure 22, Page 39.
GUN MECHANISM ASSEMBLY
The transparent side shows how the other principal components of the mechanism fit together in the slide
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Part Two-Operation of Gun Mechanism
Before automatic operation of the gun mechanism can take place, the gun mechanism must first be cocked and a round fed onto the tray manually. When the trigger catch lever is depressed, the round is rammed into the chamber and fired. The resulting recoil and counterrecoil will now cock the gun mechanism and feed the rounds, thus providing automatic operation.
A. FIRING THE FIRST ROUND
To prepare the mechanism for firing, the hand operating lever is moved to the rear as far as possible and then forward to the rear catch bracket. This movement, Figure 23, performs three operations:
It rotates the rammer cocking levers which force the rammer shoe to the cocked position where it is held by the loader catch lever.
It operates the star wheel catch mechanisms, through a linkage and catch release pistons, to permit a quarter turn of the star wheels.
It lowers the breech block, through the hand operating rod, to a position which causes the extractors to lock the block down.
A clip of four rounds is now placed in the loader and pushed smartly home, rotating the star wheels and placing the lowermost round on the rammer tray with its base in the slots of the rammer levers. The clip is automatically detached from the rounds, and ejected through the clip chute of the loader.
These rounds press back the feed control lever, disengaging the loader catch lever from the rammer shoe. The trigger catch lever now holds the rammer shoe until released by the trigger mechanism. The hand operating lever is returned to the forward catch bracket, and the firing selector lever set on AUTO FIRE. The mechanism is now prepared to fire automatically as long as at least two rounds are in the loader above the star wheels.
The illustration shows the position of the outer crank, the catch mechanisms, and the rammer cocking levers when the hand operating lever is moved fully to the rear.
The inward motion of the firing plunger in the trunnion operates the trigger mechanism of the slide and disengages the trigger catch lever from the rammer shoe. The rammer shoe, with the rammer levers and the live round, is forced rapidly forward by the rammer spring. At the end of the rammer stroke, the rammer levers are spread by the slots in the tray, and the live round is released at high velocity and catapulted into the barrel chamber.
The extractors, caught by the round, have released the breech block, allowing it to be raised to the closed position.
3. Firing the Round
As the round enters the chamber, its base comes into contact with the extractors, pulls them forward, and disengages the extractor hooks from the breech block. The torsion of the breech block closing spring rotates the crankshaft with its attached cranks. The cams on the inner cranks raise the breech block to its closed position, as shown in Figure 24. After the breech block is raised, the inner cranks continue to rotate until the cams lock the breech block in its closed position. As the breech block is raised, the pressure of the cam of the left inner crank on the outer cocking lever is released. The inner cocking lever, and thus the firing pin, is now being held by the sear. During the last few degrees of motion of the inner cranks, the lower cam on the right inner crank forces the sear inward, releasing the inner cocking lever, and thus the firing pin. The firing spring then forces the firing pin into
The breech block is shown in the closed position. Transparent treatment of the block shows the relationship of the parts of the assembly.
the primer. This is illustrated in Figure 25. The firing pin rotates the freely moving inner and outer cocking levers during its stroke.
B. FIRING AUTOMATICALLY
1. Action of Breech Mechanism and Loader During Recoil
During recoil, the cam on the side door acts on the outer crank to rotate the crankshaft and the inner cranks. The first movement of the inner cranks retracts the firing pin and unlocks the breech block, Figure 26. To do this, a
The breech block is cocked during recoil, and the firing pin held back by the inner cocking lever.
cam on the left inner crank depresses the outer cocking lever, and thereby rotates the inner cocking lever, which retracts the firing pin. As the lower arm of the inner cocking lever clears the notch in the sear, the sear spring moves the sear to the right, so that it is held in position to lock the inner cocking lever. In the event of failure of the sear spring, motion of the sear to the right is insured by a cam on the left inner crank. Locking of the inner cocking lever will occur as the breech block is closing, when the outer cocking lever is released by the cam of the left inner crank.
Further rotation of the inner cranks lowers the block into contact with the toes of the extractors, rotating the extractors in the housing. The extractors violently eject the empty case through the housing and loader, between the spread rammer levers, through the rear door, and into the case deflector.
They also lock the breech block in the open position, as shown in Figure 14, page 28.
During the recoil movement of the rammer tray, the feed pawls of the loader are raised above the next rounds, which are being held in position by the stop pawls. This motion is produced by the roller on the lower end of the feed rod being moved by the guides on the sides of the tray.
2. Action of Breech Mechanism and Loader During Counterrecoil
During counterrecoil, the breech block closing spring acts to move the breech block to the closed position, but motion of the block is stopped as the breech block is latched open by the hooks on the extractors. Consequently the outer crank is carried clear of the side door cam.
As the tray moves forward during counterrecoil, the pawls on the top surface rotate the catch heads, so that the star wheels can be rotated by the round which is fed between the star wheels and then onto the tray. The rounds in the loader are forced downward by the feed pawls, which in turn are operated by the guides of the tray. As the tray is moving forward, the rammer shoe is latched to the rear by the tray catch lever, in such a position that the base of the next live round above the star wheels will be caught by the slots in the rammer levers as the round is dropped onto the tray. The tray continues forward, and when it is about one inch from battery position, the cam on the bottom of the tray trips the rocker arm, which in turn trips the tray catch lever, releasing the rammer.
The cycle, beginning with the ramming of the live round, will be repeated if the trigger is kept in the firing position, and if the loader contains sufficient rounds, so that the loader catch lever is in the releasing position. If the trigger is kept in the firing position, but the loader contains insufficient rounds for operation of the loader catch lever, firing will cease, but will begin again immediately upon the insertion of additional rounds into the loader.
3. Action of the Recoil System
The recoil system is comprised of the recoil spring and the recoil cylinder. The recoil spring provides in counterrecoil the force necessary to return the gun mechanism to the battery position, cock the rammer, and feed a new round. The recoil cylinder controls the length of recoil and the velocity of counterrecoil.
As the barrel, housing, and tray recoil, the housing carries with it the recoil cylinder piston rod. As the piston rod is drawn to the rear of the recoil
cylinder over the throttling rod, Figure 27, liquid is forced from the rear to the front of the piston through the eight holes in the piston and through the throttling bushing. Liquid pressure forces the check valve to the rear, thereby opening the ports in the valve seat. Through these ports liquid now passes to the rear of the valve. Liquid also passes to the rear of the valve around the valve seat and through the tapered grooves inside the piston rod; through the ports in the throttling rod, over the needle valve, and through the seat. These effective flow areas set up a fluid resistance which retards recoil.
Meanwhile, the recoil spring has been placed under greater compression. When recoil ceases, the recoil spring returns the recoiling parts to battery position, and the piston rod moves forward along with the housing.
During counterrecoil, the piston rod moves forward over the throttling rod, and the check valve, under spring action and liquid pressure, masks the ports in the valve seat. Liquid at the rear of the valve seat is forced to the front of the valve seat through the grooves in the piston rod and through the bore in the center of the valve seat. The tapered grooves gradually reduce the flow space as the barrel approaches the battery position until finally no liquid can pass over the valve seat. The flow of liquid from the rear to the front of the seat is now restricted to the bore in the seat, over the needle valve, and through the ports in the throttling rod. Liquid in front of the piston is forced through the throttling bushing and the holes in the piston.
Finally, the front end of the piston enters the recess in the head of the throttling rod. This action forces the liquid between the head and piston through the space between the throttling bushing and the throttling rod. The front taper of the rod reduces the flow space as the barrel approaches the battery position. This action and the tapered grooves inside the piston rod further reduce the speed of counterrecoil.
At the end of counterrecoil, the buffer pad provides a positive stop at battery position for the recoiling parts of the gun mechanism.
The length of recoil is measured by the recoil indicator, attached to the rear door. The indicator is operated by the rear surface of the rammer tray. A length of recoil of approximately 7.2 inches is required to insure automatic operation of the mechanism. Shorter recoil may be insufficient to permit the tray pawls to operate the catch mechanisms of the star wheels, thus preventing feeding. Short recoil may also cause a jam on the tray by causing the star wheels to start to rotate before the ejected case has had time to clear them. The maximum operating length of recoil is about 8.3 inches and is limited
by the design of the throttling rod and bushing in the recoil cylinder. Normal recoil should lie within the above mentioned limits, and will vary somewhat with the elevation of the gun, and with the quantity, specific gravity, and temperature of the recoil fluid.
The velocity in counterrecoil, and thus the rate of fire, is determined by the setting of the needle valve of the recoil cylinder. The setting of this valve has no appreciable effect on the length of recoil. The needle valve is adjustable at the forward end of the recoil cylinder.
OPERATION OF THE RECOIL CYLINDER ASSEMBLY
The top sectional view shows the recoil cylinder when the gun is at battery position, and the other two views show the flow of fluid during the recoil and counter-recoil strokes. The red areas represent fluid under high pressure, while the yellow areas represent fluid under low pressure.
Figure 27, Page 49.
OPERATION OF THE RECOIL CYLINDER ASSEMBLY
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