As the gun operates, each part must do its job at exactly the right time. This complicated teamwork of parts, as the gun feeds, fires, and ejects each round, is called the cycle of operation.

The cycle of operation is a picture of your gun in action when everything is in perfect working order. Unless you are familiar with it, you will have no way of knowing what is wrong when the gun does not work.

A gun with one whole side cut away will make it possible to watch the action of the parts as the gun recoils and counter recoils during automatic fire. Then, with this action as a background, we can follow one round through the gun and see what happens to it.


The force of the propelling charge, which shoots the projectile forward, also kicks the bolt backward. The bolt is locked to the barrel extension by the breech lock. So the three recoiling parts—bolt, barrel, and barrel extension—start back together.

The barrel and barrel extension speed backward with as much momentum as a small truck. The gun's first job is to stop them.

The barrel extension shank, which is always hooked to the oil buffer piston rod, immediately begins forcing the piston rod back into the oil buffer tube.

Pushing against the oil, the piston head and valve are forced together, closing the six holes in the piston head. Now the only way the oil can escape to the other side of the head and valve is through two narrow slots, called restricted openings, on each side of the head and the valve—and the piston rod head can move only as fast as the oil escapes.

This hydraulic action, helped by the oil buffer spring, absorbs the recoil of the barrel and barrel extension. They are almost stopped by the time they hit the front of the oil buffer body.

While this is going on, the next job is to unlock the bolt from the barrel extension so the bolt can pull the empty case from the chamber and leave space to feed the next round into the chamber.



Separating the bolt and barrel extension.


As the barrel extension travels back, the breech lock pin hits the slanted surfaces of the breech lock depressors, forcing the breech lock down just as it moves' off the breech lock cam. The bolt is completely unlocked from the barrel extension at about % inch of recoil, and begins pulling the empty case from the chamber.

In order to complete the recoil and counter recoil action as rapidly as possible, the next job is to speed up the bolt.


Accelerating the bolt

The barrel extension hits the rounded front of the accelerator, turning it backward. The accelerator tips hit the lower projection on the bolt, giving the bolt a powerful boost to the rear. In this way the accelerator serves as an energy transmitter. It also acts as a timing device, because the transfer of energy slows down the barrel extension and speeds up the bolt.

At this point the barrel and barrel extension have almost finished the IVs inches of their recoil, but the bolt still has to go all the way to the back plate. The next job is to hold the barrel extension back at the end of its recoil and make it wait for the bolt, so that all the recoiling parts will return to battery position at the same time.



Lockinig the barrel extension back.
As the accelerator turns backward, its locking lugs move down until the shoulders of the barrel extension shank are clamped between the locking lugs and the underside of the accelerator. The accelerator thus acts as a locking device, holding the barrel extension against the oil buffer body at the end of its recoil stroke. The oil buffer spring is locked back in a compressed position.

Stopping the bolt


The bolt continues to travel to the rear, pushing against its double driving spring, which smooths out its action. At the end of the recoil stroke—after the bolt has traveled back about 7 1/8 inches —it hits the buffer plate and jams it against the buffer disks, which absorb the force of the blow.


During the bolt's recoil, the gun is also being cocked. This involves two main steps: the firing pin must be drawn back against its spring, and the notch on the firing pin extension must be hooked to the sear notch to hold the firing pin back. In addition, there must be some way of preventing the gun from firing too soon if the firing pin notch should accidentally slip off the sear notch.



Pulling back the firing pin.

When the gun is in battery position, the top of the cocking lever is resting on the rear cocking lever cam—the back half of the V-slot. As the bolt begins to move back, the top of the cocking lever is forced forward and its lower end is pivoted backward.

This lower end sticks through a slot in the firing pin extension. Its heel presses the firing pin extension backward, compressing the firing pin spring against the sear stop pin.

Locking the notches


A little farther back, the firing pin notch overrides the sear notch, forcing the sear down. The sear spring forces the sear back up, and the two notches are interlocked.

The cocking lever continues to push the firing pin extension back a little farther than is actually necessary, to make sure the two notches will engage even if the heel of the cocking lever or the camming surface of the firing pin extension is worn. This extra distance is called overtravel.

The cocking lever also makes it impossible for a round to be fired too soon, even if some defect in the gun should accidentally release the firing pin during recoil or counter recoil. As long as the heel of the cocking lever is in the path of the firing pin extension, the pin cannot go all the way forward even if it slips off the sear notch. This protection, called fire control, is in effect as soon as the bolt has moved back from battery position a short distance.




At the end of recoil, the bolt hits the buffer plate hard enough to bounce forward into battery position without help. Its driving spring serves chiefly to smooth out the counter recoil action.

As counter recoil starts, the barrel and barrel extension are still locked back, waiting for the bolt. The first job is to unlock them at exactly the right time.


Releasing the barrel extension

At the start of recoil, the tube lock in the oil buffer body is keeping the accelerator tips from bouncing up too soon and catching in the breech lock recess of the bolt. But as the bolt speeds forward, its lower projection hits the tips and turns the accelerator forward.

This turns the locking lugs back out of the way and releases the barrel extension, which is pushed forward by the oil buffer spring and starts its own counter recoil.

The front of the accelerator also pushes against the barrel extension and gives it an additional boost forward. This time the accelerator slows down the bolt and speeds up the barrel extension—just the opposite of its job on recoil.

Opening the valve


As the barrel extension starts forward, it pulls the piston rod head and valve through the oil buffer. But this time the valve drags behind the piston head. The oil can flow through the six holes—called throttling ports—in the piston rod head, and through the space at the center of the valve, as well as through the restricted openings at the sides. This permits faster forward movement of the barrel and barrel extension.

The bolt and barrel extension are now nearly together, with the bolt still traveling a little faster than the barrel extension. The next job is to lock them together.



Lockinig the bolt to the barrel extension

The bottom of the breech lock hits the breech lock cam on the bottom of the receiver and is forced up into the breech lock recess of the bolt, which has moved over it just in time.

The bolt, barrel extension, and barrel are now locked together. They are approximately % inch away from battery position, and they travel the rest of the way together.


Cocking action continues all through counter recoil. The tip of the cocking lever hits the front cocking lever cam—the forward half of the V-slot—and starts pivoting backward. This moves the heel of the cocking lever forward until overtravel is eliminated and the firing pin notch is held back only by the sear notch.
Withdrawing fire control


Fire control is even more important on counter recoil than on recoil, because a live round is now in the T-slot. To prevent the round from being accidentally discharged outside of the chamber, the heel of the cocking lever remains in the path of the firing pin as the bolt continues to travel forward. If the firing pin notch somehow slips out of the sear notch, the heel of the cocking lever stops the firing pin before it can spring forward to fire the round.

The cocking lever heel gradually moves forward out of the way. When the bolt is almost back in battery position, fire control ends. The gun is ready to fire again.
While all this recoil and counter recoil movement is going on, the gun is performing six major functions on each round:

1. Feeding it into the feedway.
2. Extracting it from the belt.
3. Feeding it into the chamber.
4. Firing it.
5. Extracting it from the chamber.
6. Ejecting it.



Feeding into the feedway
12. When the bolt is forward in battery position, the belt feed slide is in the cover; the ammunition belt is held by the belt feed pawl and belt holding pawl.
13.As the bolt recoils, its cam grooves guide the belt feed lever lug, pivoting the lever and moving the slide out the side of cover. The belt is held stationary by the belt holding pawl while the belt feed pawl pivots, compressing its spring, and rides up over the link holding the next round.14.At full recoil, the belt feed slide moves out enough to permit the belt feed pawl spring to snap the pawl down behind the new round.



Feeding into the feedway
15. As counter recoil starts, the belt feed lever begins moving the slide back into the receiver. The belt is pushed in by the belt feed pawl. The next round rides over the belt holding pawl, forcing it down and compressing its spring.

16.When the bolt is back in battery position, the slide is all the way back in the gun. The belt holding pawl is snapped up behind the second round by its spring.
17.If for any reason the round that was in the feedway was not extracted from the belt, the belt feed pawl arm rides over it, holding up the belt feed pawl to prevent double feeding.




Feeding from the belt
18.In battery position, the cover extractor spring holds the extractor hook firmly in the extracting groove of the new round. The ejector presses against the side of the round. When the bolt begins to recoil, the extractor hook pulls the new round from the ammunition belt.
Feeding into the chamber
19.As the bolt moves farther back, the cover extractor cam forces the extractor assembly downward, pushing the new round down into the T-slot. The ejector supports the side of the round to keep it from falling through the slot. The extractor lug rides along the top of the extractor switch, forcing the rear of the switch downward. Near the end of recoil, the lug moves off the end of the switch, which snaps back up into position.
Feeding into the chamber
20. At the start of counter recoil, the camming surface on the bottom of the extractor switch forces the extractor assembly farther down. The extractor stop pin on the left side of the bolt keeps the assembly from going too far down, and the new round thus is lined up with the chamber.



21.When the new round is part way in the chamber, the extractor lug hits the extractor cam, forcing the extractor assembly up over the next round in the feedway. The face of the bolt, ending its counter recoil stroke, pushes the round all the way into the chamber.

When the trigger is pressed, it raises the rear of the trigger bar, pivoting the front end down against the sear tip.

The sear notch releases the firing pin notch and the firing pin snaps forward. The sear spring forces the sear back up into position.


The front end of the trigger bar remains down as long as the trigger is pressed. In automatic fire, when the bolt is almost back in battery position, the sear tip rides under the forward tip of the trigger bar and is cammed down to fire the gun.

Automatic firing continues as long as the trigger is held down, and stops when the trigger is released. Any number of rounds, from one shot up to long bursts, can be fired.




Extracting from the chamber
24.As soon as the bolt is unlocked from the barrel extension on recoil, the T-slot begins pulling the empty case out of the chamber. The case is free of the chamber before the end of recoil.
25.As the new round is forced down toward the center of the T-slot at the start of counter recoil, it pushes the empty case out the bottom of the gun. The last empty case in an ammunition belt is pushed out by the ejector.


Some of these six functions are being performed on different rounds
during each recoil and counter recoil stroke.

In battery position (Diagram 22, Page G-61):
The red round is fired.

On recoil (Diagram 24):
The round outlined in red is being extracted from the belt.
The empty case of the red round is being extracted from the chamber.

On counter recoil (Diagram 25):
The empty case of the red round is being ejected.
The round outlined in red is being fed into the chamber.
The gray round is being fed into the feedway, against the cartridge




To keep the caliber .50 working properly, four adjustments must be kept accurate. The gun must be adjusted for headspace and timing. The oil buffer and the buffer adjusting screw must be set properly.

The purpose and method of these adjustments are described in the next ten pages. Also shown here is another set of adjustments which must be made in changing the direction of feed. In most cases, diagrams in this section are exaggerated for the sake of clarity.



Although there are other more technical definitions of headspace, for all practical purposes it is simplest to think of the headspace of the caliber .50 machine gun as the distance between the face of the bolt and the breech end of the barrel, when the bolt has been pulled back enough to prevent any play at the breech lock.

This distance, which can be measured with a headspace gage, is regulated by the distance the barrel is screwed through the barrel extension.

Headspace distance

When headspace is too tight, the bolt cannot operate efficiently, the action becomes sluggish, and the gun may stop—or it may not even go into battery position at all.

When headspace is correct, the gun can go into battery, the round is properly supported in the chamber, and the bolt operates efficiently.

When headspace is too loose, the parts are battered and the round is so improperly supported in the chamber that the case may be blown apart when the gun is fired.
Your machine gun is such a precision-built instrument that it cannot work properly unless headspace is correct to within thousandths of an inch. The next six diagrams, showing top and side views of the gun with varying headspace adjustments, explain why.




Excessively tight headspace
occurs when the breech end of the barrel projects so far back that the bolt's forward movement is stopped before its breech lock recess has moved over the breech lock. As a result, the breech lock cannot move up all the way into its recess, and the bottom of the breech lock cannot get up over the breech lock cam. This prevents the barrel extension from going forward into battery—when the cover is raised, you will see a space between the front of the barrel extension and the trunnion block.

The bolt may be so far back that the gun will not fire at all. At best, the gun will fire one shot and stop, because the extractor assembly cannot reach the ammunition belt to pull out a new round.

Excessively tight headspace

Tight headspace
occurs when the bolt goes forward barely enough to permit the breech lock to enter the recess.

The front of the breech lock is held tightly against the front of the breech lock recess. The bolt, which can operate smoothly only when there is a little play between lock and recess, gets off to a slow start on recoil. The action of the gun is sluggish; it may fire a few rounds and then stop. It is unable to pull a heavy ammunition belt.

The headspace gage cannot be inserted between the face of the bolt and the breech end of the barrel, showing that headspace is too tight.

Excessively tight headspace

Correct headspace
allows just a little play between the breech lock and the front of the breech lock recess.

The bolt gets off to a head start before the recess hits the breech lock, and the gun works efficiently. The round is firmly supported by the face of the bolt and the shoulders of the chamber.

Before measuring headspace with a gage, the bolt is drawn back just enough to take up the play between the breech lock and the breech lock recess. The distance between the face of the bolt and the breech of the barrel is then just large enough for the headspace gage to enter, showing that headspace is correct.

Correct headspace


G64 - 65


Loose headspace
allows too much play between the breech lock and the breech lock recess. As a result, these two parts and the breech lock cam take such a battering that they are soon unfit for service.

When the bolt is drawn back far enough to take up this play at the breech lock, a headspace gage reveals that the distance between the face of the bolt and the breech end of the barrel is now too large.

Loose headspace


Excessively loose headspace
occurs when the breech end of the barrel is not screwed completely through the barrel extension. As a result, there is too much space between the face of the bolt and the barrel, and the bolt is unable to hold the round firmly against the shoulders of the chamber. This has three damaging effects:

One: Often the cartridge case, unsupported by the shoulders of the chamber, is blown apart by the exploding charge. This may result in a separated case, with the tip blown off and left in the chamber after the rest of the case is extracted, or a split case, cracked along the sides. In combat it is difficult and sometimes impossible to clear a separated case from the chamber.

Two: Since some of the expanding gases which should drive the bullet out of the barrel escape around the breech, the range of the gun is cut down and the flight of the bullet becomes inaccurate, resulting in a less concentrated shot pattern.

Three: As the bolt slams forward into battery, it hits the barrel extension. The T-slot is battered, and frequently the barrel extension is broken about halfway back. Broken barrel extensions caused by loose headspace are one of the most frequent combat breakdowns.

Results of excessively loose headspace
Correct headspace




Checking breech lock cam nutTo be absolutely sure that headspace is correct,
two operations are necessary: the headspace
must be carefully adjusted, and the adjustment
must then be checked with a gage.
As a preliminary step, check the breech lock
cam nut on the bottom of the receiver. You
should be able to turn the nut slightly with your
fingers, and its cotter pin should be in place.
If the breech lock cam is not correctly installed,
have the ordnance or armament shop check
it at once.
There are two different ways of adjusting
One method of making the adjustment while
assembling the gun
was described oh page
It is also possible to adjust headspace after
the gun is assembled.
Prying barrel locking notches

To use this method:
(If the gun is set up for left feed, remove the right-hand rear cartridge stop assembly while making this adjustment.)

1 Pull the bolt back about an inch and let it go. If the parts do not return fully to battery position, proceed with Step 2. If they do return fully to battery position, screw the barrel in until the parts no longer return to battery without being forced. To turn the barrel, retract the bolt slightly and pry the barrel locking notches to the left with a tool or the point of a cartridge. Then . . .

Pulling bolt back and prying locking notches

Unscrew the barrel one notch at a time
until the recoiling parts will go into battery when you pull the bolt back one inch and let it go. Pry the locking notches to the right to unscrew the barrel.

3 Then unscrew the barrel two more notches. This will allow just enough play at the breech lock to prevent sluggish action.





After adjusting headspace by either of the two methods, check the adjustment with a gage before firing the gun. Two gages for checking headspace are now in use:
The old gage (Al 96228) is a one-piece combination headspace and timing gage. The end marked HEADSPACE—.200 is a GO gage for checking headspace. The end marked TIMING —.116 is a NO FIRE gage for checking timing. The new gage (A351211) is a double-ended GO—NO GO headspace gage. It comes attached to two timing gages, FIRE (A351214) and NO FIRE (A351213), to make up head-space and timing gage assembly A351217. One advantage of the new gage is that it can be inserted through the bottom of the receiver to check headspace in turrets where the cover of the gun cannot be raised all the way.

Headpace gages
To check headspace with a gage:
1 Cock the firing pin by charging the gun and letting the parts go fully into battery.
2 Back off the bolt slightly—not more than 1/16th of an inch—to relieve the pressure of the driving spring and take up play between the breech lock and breech lock recess. Then...

3 Check for tight headspace
by inserting the headspace end of the gage into the T-slot, between the face of the bolt and the breech end of the barrel.
4 If the gage does not go in, headspace is too tight; unscrew the barrel one notch at a time until the gage just goes in without being forced.
5 If the gage does go in easily the first time, check for loose headspace by screwing the barrel in, one notch at a time, until the gage will not enter. Then unscrew the barrel one notch so the gage can enter.

3 Check for tight headspace
by inserting the GO end of the headspace gage into the T-slot, between the face of the bolt and the breech end of the barrel.
4 If the gage does not go in, headspace is too tight; unscrew the barrel one notch at a time until the gage just goes in without being forced.
5 If the gage does go in easily the first time, check for loose headspace by trying to insert the NO GO end of the gage into the T-slot. If this end of the gage does not go in, headspace is correct. If it does go in, tighten the barrel one notch at a time until the NO GO end will not enter. When headspace is correct the GO end of the gage must just go in and the NO GO end of the gage must not go in.

6 Remove the gage and release the firing pin.

6 Remove the gage and release the firing pin.


No method of setting headspace will always give correct headspace adjustment. The adjustment should always be checked. A fairly accurate check can be made without a gage.
1 Charge a dummy round into the chamber. Raise extractor and pull back on it. There should be an extremely small amount of rearward motion of the bolt before the barrel extension starts rearward. This independent rearward movement of the bolt should not be more than the slightest possible movement.
2 If no dummy is available follow the same procedure. The independent rearward movement of the bolt should be slightly greater than when a dummy is used but it should be very slight.
3 With practice these methods of checking headspace are very accurate.





Timing adjustment

In automatic fire, the firing pin should be released when the parts are a fraction of an inch out of battery position. To be exact, the gun should be less than 0.116 of an inch out of battery, but not less than 0.020 of an inch out of battery, when it fires.

Results of incorrect adjustment
If the gun is 0.116 of an inch or more out of battery when it fires, it fires too early. It will fire two shots and then stop, because the recoil from the second shot starts before the extractor hook can reach the cartridge in the feedway. If the gun is less than 0.020 of an inch out of battery when it fires, it fires too late. The rate of fire is slowed down. The barrel extension batters the trunnion block as the parts slam into battery position.


Checking the timing

1. Adjust and check headspace.
2 Cock the firing pin by charging the gun and letting the parts go fully into battery.
3 Retract the recoiling parts about a quarter inch. Then . . .
Checking the time with old gageChecking the time with new gage

With the Old Gage

4 Check for early timing by inserting the timing end between the front of the barrel extension and the trunnion block, with the curved part of the gage resting over the barrel. Let the barrel extension close slowly on the gage. Then press the trigger. If the firing pin is released, firing is too early.

5 If the firing pin is not released, check for late timing by obtaining a 0.020-inch feeler gage and inserting it between the barrel extension and trunnion block. If the firing pin does not release, firing is too late.

With the New Gage

4 Check for early timing by inserting the NO FIRE leaf of the gage assembly between the front of the barrel extension and the trunnion block. Let the barrel extension close slowly on the gage. Then press the trigger. If the firing pin is released, firing is too early.

5 If the firing pin is not released, check for late timing by inserting the FIRE leaf between the barrel extension and trunnion block. If the firing pin does not release, firing is too late.

If you find that the timing is too early or too late, try installing different trigger bars until you find the one that works. If you are unable to obtain the correct timing this way, don't try to file or bend a trigger bar—take the gun to ordnance for the necessary repairs.




When you turn the oil buffer tube, the valve inside the tube also turns because the valve key is held in a slot in the side of the tube. Turning the valve changes the size of the restricted openings. Increasing the size of the openings lets the piston push back through the oil more rapidly on recoil. Decreasing them slows up the piston.

The openings are largest, and the rate of fire is most rapid, when the slots in the side of the valve are in line with the slots in the side of the piston head. This is the proper adjustment for aircraft machine guns, regardless of temperature and altitude, in all cases where the buffer tube had been filled with the correct oilt (Recoil Oil 2-36, latest issue)

Results of improper adjustment

If the restricted openings are too small, the rate of fire will be too slow. The gun's action may stop part way back on recoil.


Making the adjustment

Because of differences in the manufacture of machine guns, the openings are not necessarily at their widest when the arrow on the buffer tube points to the O (for OPEN). The only way to be sure the adjustment is correct is to see that the tube lock protrusion is in the third notch from the left at the bottom of the tube. Because it is difficult, if not impossible, to make this adjustment when the gun is assembled, remove the oil buffer group and put it on the table. Then, even if you are blindfolded or working in the dark, it is easy to turn the tube to the left (counterclockwise) until the clicking stops, then turn it to the right (clockwise) until you hear three clicks.
Adjusting the oil buffer


Emergency check

T he oil buffer can be roughly checked in combat without removing the back plate of the gun or its adapter. Simply squint through the hole in the back plate and check the angle of the slot on the rear of the oil buffer tube. If the slot is in the same position as the two hands of a clock at 5 minutes to 5, the setting is approximately correct.




Tightening the buffer adjusting screw increases the pressure on the buffer disks; loosening the screw decreases the pressure. There are usually 22 disks in the tube. If they are too worn, or if there are too few of them, the adjusting screw will go in too far. If there are too many disks, or if they are not seated properly, the screw will not go in far enough.

Results of improper adjustment

If the screw is too tight, or if the disks are too worn, they are unable to cushion the shock of the recoiling bolt. The pounding of the bolt may strip the threads off the adjusting screw and drive it out the rear of the buffer tube, or shear off the sides of the back plate and force it off the back of the receiver. If the adjusting screw is too loose, or if there are too few disks, the bolt will batter against the back plate instead of the buffer plate, and may break the back plate and drive it off the receiver.

Making the adjustment

Adjusting buffer screw

Tighten the buffer adjusting screw as tight as you can, using a caliber .50 combination wrench if one is available. Remove the back plate and press on the buffer plate to make sure it is held firmly in position.
The adjustment is correct when the end of the screw does not go inside the tube, nor stick out more than one thread.
If the adjusting screw goes in so far that its end is inside the buffer tube, the disks have become worn and flattened, or too few disks have been inserted. Instead of adding one new disk yourself, have ordnance check the condition of the disks and replace them all if necessary.
If, after the screw is tightened as much as possible, you can see more than one thread, make sure that the disks and buffer plate are properly seated, and that you have not inserted too many disks. If the screw still sticks out too far, have ordnance make the necessary repairs.




Change the bolt switch. Lift out the extractor assembly and turn the bolt switch. For left hand feed, the narrow end of the switch goes to the rear, providing a clear track along the cam groove marked L For right hand feed, the wide end of the switch goes to the rear, providing a clear track along the groove marked R.

Change the belt holding pawl. It should go on the same side as the direction of feed —right side of the receiver for right feed, left side for left feed.
Change the cartridge stops. Since the rounds are fed in against them, they go on the side opposite the direction of feed. For right feed, put the front and rear cartridge stops and the link stripper on the left side of the receiver. For left feed, put the front cartridge stop and the right-hand rear cartridge stop assembly on the right side of the receiver.

Changing the direction of feed
belt feed lever plunger and plunger spring.
Change thebelt feed pawl and pawl arm.
belt feed lever plunger and plunger spring.
1.Remove the belt feed lever and belt feed slide. Take the belt feed pawl and pawl spring off the slide by drifting out the pawl pin.
2.Take the pawl arm off one side of the pawl, turn it over, and place it on the other side. When you hold the pawl with the smooth side toward you and the pointed end facing the direction of feed, the pawl arm should be on top.
3.Turn the pawl around, put the pawl spring on its stud, and replace the pawl in the slide, with the pointed end of the pawl at the rounded end of the slide. Insert the slide in the cover with the pawl arm on top.
4.Replace the belt feed lever plunger and plunger spring—in the upper hole for left hand feed, or lower hole for right hand feed. Replace the belt feed lever on its stud, and make sure the plunger and spring hold the slide in the cover.





The parts of an aircraft machine gun are precision built to fit as closely as possible, yet they must work together at high speeds and in extreme ranges of temperature. Dirt, moisture, or grease between the moving parts—even in very small quantities—causes sluggish guns, stoppages, and broken parts. Small burrs — scratches or gouges in the metal—also can interfere with the gun's operation.

Inside the barrel, heat, pressure, and friction produce tiny cracks—only a twelfth as deep as the thickness of a hair and almost impossible to see—in the surface of the bore. Although a carelessly cleaned bore may look all right, the cracks will retain moisture, the waste left by the exploding gases, and smears of metal from the jackets of the bullets. The result: rust. Explosion of the propelling charge leaves only an almost harmless ash. But the exploding primer mixture leaves a salt—potassium chloride—similar to ordinary table salt. This primer salt, which quickly absorbs moisture from the air, is deposited in the chamber and bore, in the recesses of the bolt, and around the front part of the receiver.

These parts will rust very quickly if primer salt is left on them. The salt is not affected by oil; if a gun is oiled without being carefully cleaned, the salt continues to absorb moisture even when covered with oil.

Therefore the gun must be cleaned, and cleaned thoroughly, immediately after every mission.

Since aircraft guns are only lightly oiled, condensation of moisture will cause rusting even of a perfectly clean gun.

Therefore every gun of every aircraft should be cleaned every day, whether it has been used or not.


Field strip the gun and detail strip the bolt, oil buffer, barrel, and cover groups. Remove the extractor switch, switch spring, belt holding pawl, and pawl springs from the receiver. For the bore of the barrel, the best cleaning agent is rifle bore cleaner (Spec. No. RIXS205) —a combination solvent and preservative issued for combat use. If that is not available, other good cleaning solutions are hot water and G.I. soap, or hot water and soda ash (1 1/2i tablespoonfuls of soda ash to a pint of water). Lacking any of these, use hot water alone—or, if necessary, even cold water. "White spirits" and kerosene are frequently used in combat theaters.

With any of these solutions, put the muzzle into a bucket or vat of the fluid. Attach a clean flannel patch to a cleaning rod and move the rod back and forth through the barrel from the breech end for about a minute, pumping the cleaner in and out.

If a brass or bronze wire brush is available, run it through the barrel three or four times while the bore is still wet. Pump the cleaning fluid through the barrel again to clean it. Dry the cleaning rod and use clean, dry flannel patches to swab the bore until it is perfectly dry and clean. Dry and clean the chamber, using a patch on a pointed stick if necessary. Finally, pull a clean patch smoothly through the bore and chamber, letting the cleaning rod and patch turn with the rifling.

The bearing surface of the barrel—the flat part that slides in the front barrel bearing—should be cleaned and shined with crocus cloth to remove deposits and prevent binding. In an emergency, a fine oil stone can be used instead of crocus cloth.

Clean other parts of the gun with rifle bore cleaner, dry cleaning solvent (Spec. No. RIA-ES12), "white spirits" or kerosene. Thoroughly clean every part with a brush or rag to remove all dirt, moisture, and primer salts. Clean and flush the recesses in the bolt, using a long brush or a rag on a pointed stick. Dirt, moisture, or primer salts left in the firing pin



Cleaning equipment

port will result in "light struck primers"—the firing pin will be slowed down and will not dent the primers enough to fire. Other parts of the bolt requiring special attention are the hole for the driving spring, the T-slot, the breech lock recess, and the recesses at the rear of the bolt.
Wash the back plate group and oil buffer tube assembly with the cleaning solution but do not soak them, since the fluid will damage the buffer disks and the oil buffer packing. Wipe off the casing, being careful to remove all deposits from the extractor switch recess, the area around the breech lock cam, and the front barrel bearing.
After the parts have been cleaned, dry them with a clean, lintless rag. Don't touch them with your bare hands until they have been oiled-fingerprints contain an acid which causes corrosion. If cloth gloves are available, wear them while oiling the gun. If gloves are not available, keep your hands covered with oil while handling the parts.
When a new part is issued, examine it carefully for any traces of cosmoline, a heavy grayish-brown rust-preventive compound occasionally used in shipping guns and parts. Have ordnance remove any cosmoline you find.
Cleaning the barrel




Removing rustRemoving rust
Any rust, especially at points where metal parts rub against each other, will prevent smooth operation. Remove it with crocus cloth or similar material. If the rust is pitted, remove it with a fine oil stone obtained from your crew chief or from armament or ordnance. Don't use sandpaper, carborundum, or a coarse stone.
Removing burrsRemoving burrs
Any burrs or scratches must be polished off, especially where metal surfaces are in contact. First use a medium oil stone to remove the burr. Always rub in the direction the part has been worn, instead of working crosswise. Then use a finer stone to smooth out the surface and give it a polish.

Oil must be applied carefully to avoid any "freezing" in the extremely cold temperatures of high altitudes.
If the gun is assembled dry, sub-zero temperatures will produce a metal-to-metal freeze almost impossible to break apart. Too much oil is just as bad, for it picks up moisture which

freezes inside the oil and locks the moving parts together.
When oil is applied properly, there is just a film over each part. Even if particles of moisture freeze inside the gun, the oiled parts will slide over the ice like skates.
Regardless of weather conditions at ground level, oil the gun with O.S. 1361, a special lubricating oil for aircraft instruments and machine guns. To get the right amount of oil on the gun, wipe each part with a rag that has been well-oiled with O.S. 1361. Oil the barrel bore and chamber by swabbing them with a well-oiled patch.
If the gun is to be out of service for a few days, use engine oil SAE 30 to protect the barrel. For longer storage, use a light rust preventive compound instead of oil. The gun must be thoroughly cleaned and re-oiled with O.S. 1361 before being used again.



Desert Precautions

In theaters where airfields are dusty and sandy, special care must be taken to keep sand and dirt out of the cleaned guns. Your gun should never be left in the bomber between missions. If it is necessary to leave it in, be sure it is well covered. As soon as the gun is mounted in the plane, put tape over the muzzle end of the barrel and leave it there until after the take-off; the first shot will get rid of it. If possible, take a little kerosene along on a mission. If your gun gets so dusty during the takeoff that it will not operate correctly, pour a little kerosene on the working parts to clean them. This method must not be used, however, in any theater where extremely cold temperatures may be encountered.

Filling the Oil Buffer Tube
Although ordnance is responsible for the maintenance of oil buffers in most combat theaters, refilling is a simple operation if you should ever have to do it yourself.

The only correct oil is A/C Spec. Recoil Oil 2-36, latest issue. In an emergency use hydraulic fluid (A/C Spec. 3580), but don't take chances with O.S. 1361 or any other oil. Remove both filler screws. If they are tight, insert a screwdriver in the slot and tap it with a mallet.

Two methods can be used for filling the tube.
Use a small oil can filled with 2-36 oil. To avoid getting air bubbles in the buffer tube, turn the can upside down and press the bottom to start the oil flowing before you insert the nozzle. Remove the nozzle before you release the bottom of the can, then press it again to start the oil flowing and reinsert the nozzle in the tube. Repeat the operation until the buffer tube overflows.

Filling the tube, method 1

2. Insert a clean wire through one of the holes and let oil run down the wire from a container until the tube overflows.

In either case, tap the side of the oil buffer tube with a screwdriver several times to dislodge any air bubbles.

Before you replace the filler screws, look at the tips of the screws to see if they have been grooved all the way around where they press against the oil buffer tube. If the groove is not worn completely around the screw, it will allow oil to leak—replace the screw.

Filling the tube, method 2



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Transcribed and formatted by Thomas Wildenberg for the HyperWar Foundation