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  As robust as the cal..30 Browning machine guns are, they are not immune to malfunction. I came upon this 1953 printing of the TM9-2210 SMALL ARMS ACCIDENTS MALFUNCTIONS AND THEIR CAUSES and thought the section on the cal..30 machine guns worth sharing.
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CHAPTER 11

CAL. .30 MACHINE GUNS


Section 1. GENERAL

53. Characteristics
  Cal..30 Browning machine guns are link or web belt fed, recoil operated, and water or air-cooled. The functional mechanisms of various models in current use by the U. S. Army are similar, but differ in details. This type weapon is designed to fire at materiel, aircraft, personnel, or vehicles, according to model; to fire from tripod, bipod or antiaircraft mounts or from various vehicle mounts or aircraft; and can be adapted to either fixed or flexible mountings.

54. Operating Instructions
  The accidents and malfunctions described in this chapter result chiefly from mishandling by persons using the weapons. FM 23-55 and TM 9-205 contain instructions for operating cal. .30 machine guns.

Section 11. PRECAUTIONARY MEASURES

55. Maintenance and Inspection
  Practice preventive maintenance at all times. Maintenance and repair of cal..30 Browning machine guns consists largely in the replacement of worn or broken parts.

56. Danger Points
 a. Check all ammunition.
    (1) Prior to release for issue and use, check ammunition for proper type and grade. Refer to TB 9-AMM 4 for all small arms ammunition grades.
    (2) Before loading ammunition into the weapon, check for dirt, oil, grease, malformation, and other defects. Do not expose ammunition to direct sun rays or other high temperatures or to prolonged submersion in water.
 b. Check the bore prior to firing, to be sure that it is free of foreign matter or obstructions. If a misfire occurs, make sure that the bullet is not lodged in the barrel.
 c. When firing blank cartridges, clear the -space for at least 20 yards in front of the gun in case the closing wads or paper cups fail to break up.
 d. Attachments are used to adapt the M1917A1 and M1919A4 guns for firing blank ammunition M1909. These attachments must be removed before firing ball ammunition. Always remove the muzzle attachment before removing the cartridge-stop attachment to eliminate the possibility of firing ball ammunition with the muzzle attachment still in place. (The cartridge-stop attachment prevents the entrance of a live round into the feedway.)
 e. Prior to initial use, periodically during continued use, and whenever gun barrels are changed; check and adjust head spacing of the machine gun barrel in accordance with FM 23-55 and TM 9-1205. Ascertain that barrel locking spring is properly engaged, to prevent any increase in head spacing due to rotation of the gun barrel during normal use.
 f. To obviate danger from hangfire, wait 10 seconds after a misfire then clear the weapon quickly. In the event weapon cannot be cleared quickly and the barrel is hot, danger of cook-off exists. Keep round locked in chamber, point weapon in safe direction (for personnel and property) and allow weapon to cool before removing misfired round.

Section 111. EFFECTS AND PROBABLE CAUSES OF ACCIDENTS AND MALFUNCTIONS 

57. General
The principal causes of accidents and malfunctions of cal..30 machine guns are defective parts or ammunition, obstruction of the bore, and improperly adjusted head space.

58. Typical Cases
 a. Excessive Head Space. This condition frequently causes trouble. There is a series of notches (serrations) around the breech end of the barrel in which a spring fits for locking the barrel into place. The spring sometimes sticks on the end of the barrel before slipping into the notches and thereby leaves too much head space. TM 9-1205 gives instructions for adjusting head space. When positioning the barrel, check to see that the notches are actually on the spring instead of merely against its end. Figure 23 illustrates typical effects of excessive head space on fired cartridge cases.
    (1) Case "1" in figure 23 is that of a standard ball cartridge fired in a gun with standard head space of 1.942 inches. The appearance of the case is normal.
    (2) Cases "2," "3", and "4" in figure 23 are those of ball cartridges fired in guns having excessive head space. The gun in each instance was undamaged. Note the deformation of the forward portion of each case: the lengthening of the first cone and shortening of the neck. Such case deformation occurs only when the case lies to the rear of its fully chambered position.
 (a) Case "2" in figure 23 exhibits the effect of Y16-inch excess head space with ammunition loaded to standard pressures. Note the shortened case neck and the annular bulge formed on the first cone of the case immediately to the rear of the chamber.

  (b) Case "3" in figure 23 is that of a standard ball cartridge fired in a gun having Y4-inch excess head space. Note the shortened neck and the separation of the ease approximately one-half inch from the head.
  (c)
Case "4" in figure 23 shows the effect of firing a high pressure cartridge in a gun having Y16-inch excess head space. Note evidence of excessive head space in the separation approximately one-half inch from the head of the case and in the shortened neck. Also, note evidence of high pressure in the heavy marking of the case by the chamber wall and in the relative sharpness of the newly formed shoulder crest joining the first and second cones.

b. Defective Ammunition.
    (1) Cartridge Case With Split Neck. Figure 24 exhibits damage to a gun from a round having a case with split neck.
  (a) A live round with a split case neck was chambered in the gun. For some reason the bolt was retracted, extracting the case, but leaving the bullet in the chamber. The extracted case was ejected (1, fig. 24) and a second live round chambered. When the chambered cartridge was fired the case ruptured (2, fig. 24). The explosion ignited the cartridge lying directly above in the feedway, rupturing the case, and releasing additional gases into the action (3, fig. 24).
  (b) Note the bent cover of the gun, the bent breech lock cam, and the impressions on the cartridge case from the wall of the chamber. The presence of unburned power grains in the action further indicates that a live case was extracted, leaving the bullet in the barrel.
    (2) Hangfire.
  (a) Figure 25 shows damage to a gun resulting from a hangfire. In an attempt to clear the defective round from the gun, the action was retracted by hand sufficiently to unlock the breech bolt and begin extraction of the round from the chamber. At this point in the operation, the delayed ignition of the defective round occurred. The resulting explosion separated the case head from the case body. The escaping gases bent the cover of the gun upward.
  (b) Cartridge "1" (fig. 25) is a normal cal..30 live round. Cartridge "2" in figure 25 exhibits the deformed case body of the chambered round. Note the shortened neck. Cartridge "3" in figure 25 is a damaged round that lay in the feedway at the time of the explosion.
    (3) Excessive pressure.
  (a) Figure 26 illustrates damage to a gun caused by a high chamber pressure when firing a cartridge. A cartridge that develops excessive pressure will rupture the head of its case. In this instance it also broke the extractor and bent the cover of the gun.
  (b) In all instances of high chamber pressure, the fired case will be heavily marked by the walls of the chamber.
  (c) Accidents of this type should be attributed to excess chamber pressure only after careful examination of the evidence shows no other cause.
c. Bore Obstruction.
 

(1) An obstruction in the bore immediately forward of the chambered round may rupture the head of the cartridge case (fig. 27). An obstruction in the long middle portion of the bore may bulge the barrel at the point of the obstruction. If the resistance is sufficiently great, the bulge may become a rupture (fig. 29). An obstruction near the muzzle may bulge or split the barrel at the point of the obstruction.
    (2) A bullet lodged in the bore immediately forward of the chambered cartridge will rupture the head of the cartridge case (fig. 28). A bullet lodged in the middle portion of the bore ordinarily does not rupture the fired case. The case may show signs of high pressure. Ordinarily, the rupture of the case head of the chambered round will ignite the cartridge lying directly above in the feedway. Typical damage from these explosions consists of a fractured "T" slot, a cracked or broken extractor, and a bent cover.
    (3) Grease or heavy oil in the chamber invariably produces a flute in the fired case (fig. 30). Note the characteristically elongated and radially contoured shape of these flutes. Case heads will show some indications of high pressure. Figure 30 also shows damage to the gun which commonly results from firing with grease in both the chamber and the breech end of the bore. The high pressure ruptures the case head, releasing the powder gases into the action. Note the fractured "T" slot, cracked barrel extension, and bent cover.
d. Fractured Firing Pin.
Figure 31 exhibits damage to a gun caused by premature firing of the incoming round by a fractured firing pin. This typical accident was caused by fracture of the firing pin during counterrecoil. The firing pin spring drove the firing pin point prematurely into the primer of the incoming round. The explosion of the cartridge chipped the "T" slot and bent the cover upward.
e. Short Barrel-Plunger Spring. Some difficulty has arisen from a change that was made in the length of the barrel plunger spring for cal..30 machine guns. Springs for these guns manufactured prior to 1941 were longer than those manufactured for the weapons after that date, and the new-type shorter springs functioned satisfactorily in the old weapons except for a decreased rate of fire. The specification given in TM 9-1205 did not indicate the changed length but have since been revised to show the shorter barrel plunger spring length.
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