Showing trigger mechanism automatic firing. Toe of the connector engages under the connector stud on the change lever, preventing the connector from rising sufficiently high to be cammed forward from under the fore end of the sear; head of the connector holding up fore end of sear depressing rear, preventing the rear end of sear from engaging in the sear notch of the slide.

(1) Automatic (A).-When so set the sear is depressed as long as the trigger is held back and the piece will continue firing until the magazine is emptied.

(2) Semiautomatic (F).-When so set the sear is depressed, thereby disengaging the sear and sear notch when the trigger is pulled, but the mechanism is so constructed that the sear rises and engages in the sear notch when the slide comes back again, and the sear and sear notch will not disengage until the trigger is fully released and then pulled. With this setting the piece fires one shot for each pull and release of the trigger.

(3) Safe (S).-When so set the sear can not be released from the sear notch by pulling the trigger.

b. The action of the trigger mechanism is taken up in phases, and should be followed on the mechanism itself as the explanation proceeds. Have the trigger guard stripped completely. Study the shape of the change lever and note the following:

(1) It is a bar about 4 of an inch in diameter.

(2) It has three shallow longitudinal slots cut on top of the bar, as the handle is held vertically.

(3) The side of the bar is slotted in such a way as to leave a little tongue of metal in the center and at the lower edge of the slot.

19. Setting change lever.-a. Assemble the change lever and spring to the trigger guard. Note that the toe of the change lever spring is seated in one of the longitudinal slots on the change lever, and that as the lever is turned from one position to another it seats in the other slots. The only function of the spring and the longitudinal slots is to hold the change lever in the position in which it is set.

b. Assemble the trigger and pin to the guard.

c. Turn the change lever to rear or safe position. Note that in this posi

tion the slot is turned slightly upward, and that the full surface of the bar is on the bottom. Pull the trigger. Note that the rear top end of the trigger is slotted longitudinally, and that the metal on each side of the slot forms two shoulders which rise against the bottom of the change lever bar.

d. Push the change lever over to the vertical position, which is the automatic setting. Pull the trigger as before and note that the slot in the change lever is turned to the front, and that the two shoulders of the trigger, which before engaged the full surface of the change lever bar, now are free to pass up into the slot of the change lever; also that the tongue of metal on the bottom of the change lever slot passes through the longitudinal slot in the end of the trigger.

e. Now push change lever forward to single shot position.

f. Note that now the slot is turned partially down and that when the trigger is pulled the rear end of the trigger passes up into the change lever slot; also that the tongue of metal in the bottom of the change lever slot is now turned back and does not pass through the slot in the end of the trigger as it did in the automatic position.

g. Now observe the shape of the connector. It is shaped like a boot with a toe and heel. It has a flat surface that slopes down and toward the front from the head. (Sear spring ramp.) In rear of the head the profile extends straight downward for about % inch, then slopes slightly to the rear for 0.12 inch. (Sear carrier ramp.) This last slope is used in a cam action to be explained later. Note the narrow, flat top surface of connector. Its function is to raise the forward end of sear until cammed out from under the latter.

h. Place the connector on the connector pin and set change lever to safe, pull the trigger, and note that the connector is not raised, for the obvious reason that the trigger itself can not be raised because the change lever bar is in its way.

i. Turn change lever to automatic position, pull the trigger, and note that the head of the connector is raised and held in a vertical position and can not be tipped forward. The tongue on the change lever engages the toe of the connector as the trigger is pulled and holds the connector upright.

j. Turn the change lever to single shot position, pull the trigger, and note that the tongue on the change lever does not now engage the toe of the connector and that the head of the connector can now be tipped forward.

k. Observe now the cross pin on the sear carrier, called the connector stop; also that just in rear of the connector stop and on the underside of the sear carrier is an inclined surface sloping upward in the metal which joins the two sides of the sear carrier. This surface has a cam action with the above-mentioned surface on the connector.

1. Completely assemble the trigger mechanism.

m. Note that the center leaf of the sear spring presses on the front sloping surface of the connector and tends to press the head of the connector backward. Set change lever on safe and pull trigger. Note the head of connector is not raised above the sear carrier, for reasons given previously. Therefore, the sear nose is not depressed and hence the safe position. Change over to the automatic position and pull the trigger; the head of the connector is raised and held in the vertical position, thus depressing the sear nose and

holding it in this position, which obviously gives automatic fire as long as there are cartridges in the magazine. The tongue on change lever tends to hold connector vertically and the ramp on sear carrier tends to cam connector forward. The forces exerted by these two parts on connector are opposed, hence trigger mechanism is locked when trigger has been pulled enough to release slide.

n. Set change lever for single shot; pull trigger slowly. Note that at first the head of the connector rises and thereby depresses the sear nose, which allows the slide to go forward. If the squeeze of the trigger is continued the previously mentioned cam surface on the connector comes in contact with the cam surface of the sear carrier, and the head of the connector is cammed forward against the pressure of the center leaf of the sear spring. The connector disengages the front arm of the sear and the two outside leaves of the sear spring depress it, and the sear nose is thereby raised up in the path of the slide and engages the sear notch when the slide moves back, thus allowing only one shot to be fired. When the trigger is released the center leaf of the sear spring presses the head of the connector downward and back under the forward end of the sear, so that when the trigger is pulled again the action is repeated and single shot is fired.

o. In the semiautomatic position the connector stop prevents the head of the connector being tipped so far forward that the sear spring can not push it back in place when the trigger is released. The only function of the change lever in the semiautomatic position is the limiting of the upward travel of the trigger when its upper rear shoulders strike the top of the slot in the change lever, which in this position is turned down.

20. Classes of stoppages.-Stoppages are divided into two general classes: a. Prolonged, which are due to failure of some part which can not be replaced or to a failure which can not be remedied in the field under fire, or without expert assistance or machine shop facilities.

b. Temporary, due to—

(1) Failure of some part of the rifle of which a duplicate is carried.

(2) Faulty ammunition.

(3) Improper care of the rifle.

(4) Faults of the automatic rifleman.

NOTE. This section deals entirely with temporary stoppages.

21. Causes of stoppages.-a. Dirt. (Natural fouling incident to firing and also to careless cleaning.)

b. Insufficient oil. (From failure to oil and because of oil burning up during firing.)

c. Extraneous matter in working parts. (Due to poor cleaning, brass chipped from cartridges, breakages, blown primer.)

d. Improper assembly of rifle. (Gas ports not registered, middle prong of sear spring riding on one wall of sear carrier, etc.)

e. Breakages. (Due to wrong assembly, oversize or undersize parts, burrs, incorrect heat treatment, overheating of parts incident to firing.)

f. Burred parts. (Due to improper assembly, use of force, dirt, grit, and foreign matter in mechanism.)

g. Magazine troubles. (Due to bent or dented magazines, worn magazines, catch notch, foreign matter--as blown primer-between lips of magazine and top carriage.)

h. Faulty ammunition. (Failure of primer of charge, dented cartridges.) i. Excessive play in parts. (Due to wear in stripping parts not supposed to be stripped-as removing barrel from receiver.)

j. Ruptured cartridges. (Due to excessive head space.) Head space is the distance between the face of the bolt and the shoulder of a standard steel test cartridge. If excessive head space exists when the cartridge is chambered properly, the cartridge case will be forced against the walls of the chamber at the instant of explosion. The cartridge case base will be driven to the rear, as it is not supported by the bolt. This results in rupture about 1⁄2 inch from the base of the cartridge. In effect, the action during the period of gas pressure within the cartridge case is the same as if the chamber gripped all the outer surface of the cartridge case except the part about 1⁄2 inch from the head. The pressure within the cartridge case being approximately 50,000 pounds per square inch, the ungripped portion of the case is torn from the gripped part and forced back against the face of the bolt. (The case is never pulled apart by the extractor.)

22. Method of reducing stoppages.-a. Temporary stoppages have been divided into four positions, dependent upon the position where the bolt stops. The position of the stoppage is determined by pulling the operating handle to the rear until it strikes the hammer pin.

b. Boundaries of the positions are

(1) First position, mechanism and operating handle fully closed.

(2) Second position, operating handle strikes hammer pin anywhere from fully closed to a point where operating handle plunger pin rides over raised shoulders on ribs of operating handle guide way.

(3) Third position, operating handle strikes hammer pin anywhere from second position to a point directly over "F"" of change lever setting.

(4) Fourth position, operating handle strikes hammer pin anywhere between third position and rear.