Section Description And Operation

4.0 The weapon (Fig. I) is operated by case reaction or " blow back ".

FORESIGHT COCKING HANDLE BODY RETURN SPRING BACKSIGHT

Machine Gun Trigger

BARREL CASING

TRIGGER MECHANISM

FORESIGHT COCKING HANDLE BODY RETURN SPRING BACKSIGHT

BARREL CASING

TRIGGER MECHANISM

Fig. I. Sectional arrangement

4.1 THE BACKWARD ACTION (Fig. 2). When the cartridge is fired the propellant gases exert an equal pressure against both the bullet and the cartridge case, the latter being supported by the bolt and the compression of the return spring. The gas pressure accelerates the bullet also the cartridge case and bolt in opposite directions and as the weight of the bullet is considerably less than that of the combined weight of the cartridge case and bolt, the bullet attains a much greater velocity than that of the cartridge case and bolt. When the bullet clears the muzzle all have reached their maximum velocities but the cartridge case has not yet cleared from the chamber, thus preventing the gases escaping from the breech. The cartridge case does not clear the breech until the gases behind the bullet have dispersed into the air, ensuring that pressures are down to safe limits before the breech is unsealed.

The bolt is now being decelerated by the compression of the return spring.

The empty cartridge case, held against the face of the bolt by the extractor, is carried back until it strikes the ejector and is ejected through the opening on the right side of the weapon.

Submachine Gun Bolt Extractor
Fig. 2. Backward action
Machining Rifle Bolt

When the bolt reaches the limit of its backward travel it is forced forward by the compression of the return spring. During its forward travel the bolt contacts the top round in the magazine and, guided by the magazine lips, the round is fed into the chamber. The bolt then follows up on the round, feeds it into the chamber, and fires it just before the forward movement ceases. During the forward movement of the round from the magazine, the firing pin of the bolt cannot come into line with the percussion cap of the cartridge until the round is actually in the chamber. This provides the mechanical safety for this type of weapon. Upon firing, the backward action again commences.

Sterlingsub Machine Gun
Fig . 3. Forward action

5.0 ACTION OF THE TRIGGER MECHANISM (Figs. 4, 5, 6 and 7).

5.1 SINGLE SHOT FIRE (Figs. 4 and 5). When the change lever (I) is set to the single shot position R, the inner arm of the change lever is located under the tail of the tripping lever (2).

When with the weapon cocked, the trigger (3) is pressed, the sear cradle (4) is rotated about the sear axis pin (S), the sear (6) is lowered and the bolt is carried forward by the pressure from return spring. During this movement the tail of the tripping lever contacts the inner arm of the change lever causing the tripping lever partially to rotate. Continued pressure on the trigger causes further rotation of the tripping lever until the upper arm disengages from the step on the sear. At the same time the sear plunger and spring (7) are compressed.

When the round is fired, the sear is held down by contact with the under surface of the bolt, but as the bolt reaches the end of Its rearward movement and is clear of the sear, the sear is forced upward by pressure of the sear plunger and spring. Then as the bolt is moving forward, the sear engages against the bent on the face of the bolt and holds the bolt in the cocked position.

When the pressure on the trigger is released, the rear end of the cradle rises, lifting the tripping lever and causing it to rotate about its axis pin until the upper arm of the tripping lever re-engages on the step of the sear.

The trigger must be fully released and again pressed for each single-shot.

Submachine Gun TriggerSterling Machine GunSterling Trigger

5 2 AUTOMATIC FIRE (Fig. 6). When with the change lever (I) set at automatic A and the weapon cocked, the trigger is pressed, the projection on the upper part of the trigger lifts the end of the sear cradle (4) rotating it about its axis pin (5). This depresses the sear (6) freeing it from contact with the face of the bent on the bolt, and allowing the bolt to fly forward. The movement of the sear cradle compresses the sear cradle spring (8).

The weapon will now continue firing until either the trigger is released or the magazine is empty.

When the trigger is released the sear cradle returns to its former position under the action of the sear cradle spring, the sear is raised into the boltway and contacting the bent on the bolt, holds the bolt in the cocked position.

Stirling Submachinegun
Fig. 7. Applied Safety

5.3 APPLIED SAFETY (Fig. 7, Cocked). When with the weapon cocked, the change lever (I) is set at safe S the inner arm of the change lever is positioned directly under the short arm of the tripping lever (2). When the trigger is pressed the sear cradle and sear (6) cannot be depressed because the short arm of the tripping lever is held immovable by the inner arm of the change lever.

(Fig. 7, Bolt forward). When with the bolt forward, the change lever (I) set at safe S, the weapon cannot be cocked because the sear (6) is engaged in the safety slot (9) at the rear of the bolt, and the sear cannot be depressed because it is held immovable as described in previous paragraph.

Sterling Mk4 Operation
Fig. 8. To open butt Fig. 9. To close butt

6.0 TO OPEN BUTT (Fig. 8). Hold the weapon with the left hand near the backsight with the barrel pointing towards the ground. Pull the butt plate (II) outwards with the right hand to release the butt catch (12) and swing the butt to the rear of the weapon. With the thumb of the left hand press the back cap catch (13) and snap the butt into engagement with the lugs on the back cap. Open the butt frame to form a triangle and the butt plate catch (14) will engage to lock.

7.0 TO CLOSE BUTT (Fig. 9). Release the butt plate catch and collapse the triangle by pushing the tubular member into the frame. With the thumb of the left hand press the back cap catch, at the same time push the back cap forward and swing the butt away from the back cap. Pivot the butt to its folded position, swing the butt plate out to operate the butt catch to engage in the barrel casing, then fold the butt plate flat to lock in position.

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