Remington M870 Operating Cycle
To understand an operating cycle of the M870 shotgun fully, you must know the names and general functions of the parts of the gun. To become more familiar with the parts during our discussion of an operating cycle, refer to figure 3-54A, the individual parts breakdown, and figure 3-54B, the accompanying parts list.
The entire operating cycle of the M870 shotgun is completed by pulling the trigger, sliding the fore-end rearward to open the action, and forward again to close the action. The fore-end is mounted on double-action bars and is fully controlled and operated by the shooter.
Assume the magazine is loaded and one shell is in the chamber and locked; the gun is ready to fire. The firing cycle is described in the following paragraphs.
FIRING.- with the crossbolt safety pushed to the FIRE position (red band showing), the gun is fired by pulling the trigger. The top part of the trigger rotates
Figure 3-54A.-M87O individual parts breakdown.
forward carrying the right connector, in READY position, forward against the sear. This movement pivots the sear out of engagement with the hammer. The released hammer, with force from the spring-loaded hammer plunger, strikes the firing pin, which is pinned in the breech bolt and spring retracted. The firing pin strikes the primer and ignites the powder charge. During the upward movement of the hammer, it engages the action bar lock just before it strikes the firing pin. Downward movement of the front of the action bar lock is restrained until pressure against it is briefly released by the shooter's arm as it recoils rearward When the action bar lock is released, the forward end of the action bar lock is lowered from its position at the rear of the left action bar, and the rear section rises and lifts the left connector, which lifts the right connector from contact with the sear. This completes the "lock or firing cycle. The action bar lock serves a twofold purpose. It serves as a safety feature that disconnects the trigger assembly and sear until a shell is fully seated in the chamber and the breech mechanism again is ready for firing and it locks the action closed.
After pulling the trigger, pulling the fore-end rearward will open the action and accomplish the unlock, extract, eject, cock, and feed cycles.
UNLOCKING.- The initial rearward movement of the fore-end, after the shell has been fired, carries the slide to the rear of the breech bolt. As the breech bolt passes to the rear, the slide cams the locking block from the recoil shoulder of the barrel. This movement unlocks the action and cams the firing pinto the rear where it is locked and prevented from protruding through the bolt face.
EXTRACTING.- Continued rearward movement of the fore-end opens the action. The breech bolt moves back and the fired shell is extracted from the chamber. The extractor claw, which overhangs the bolt face, grips the rim of the shell tightly as extraction progresses. Pivot pressure is exerted on the rear of the extractor by the extractor plunger and spring.
EJECTING.- AS the extracted shell clears the chamber, its base engages a shoulder on the rear of the ejector spring, which is located on the left side of the receiver. This pivots the shell so its front end is ejected first through the ejection port.
COCKING.- Before ejection occurs, the breech bolt in its rearward travel forces the hammer down against the coiled hammer spring to engage the sear. Sear spring pressure locks the sear in a notched position against the cocked hammer.
Figure 3-54B.-M870 parts list. 3-28
FEEDING.- The final movement of the fore-end carries the slide, breech bolt assembly, and locking block to the rear of the receiver. Termination of this rearward stroke also permits the left action bar to cam the left shell latch, in turn, releasing the first shell from the magazine. The released shell is forced from the magazine by a spring-loaded follower. The carrier receives the released shell. Meanwhile, the right shell latch, which was caromed into the magazine way by the right action bar during the extraction cycle, intercepts the base of the second shell.
With a shell resting on the depressed carrier, forward movement of the fore-end will close the action of the gun and complete the loading and locking cycles.
LOADING.- Forward movement of the fore-end will carry with it the slide, the breech bolt, and the locking block. The carrier dog is engaged by the slide, pivots the shell carrier upward, and places a shell in the path of the returning breech bolt. As the bolt continues to advance, it depresses the ejector spring and the shell is picked up and loaded into the chamber. The carrier dog is released by the passing slide, forced up by the carrier dog follower, and pivots the carrier from the path of the loading shell. The following shell from the magazine, being retained by the right shell latch, is released by the caroming action of the returning right action bar. At this point the shell is intercepted and held by the left shell latch until the next feeding cycle.
LOCKING.- When the shell is fully loaded in the chamber, the action closes and the bolt is against the shell base. The slide continues to travel within the bolt and cams the locking block into the recoil shoulder of the barrel. The locking block secures the breech bolt firmly and is supported by the slide as it completes its forward travel. With the locking block fully seated, the passage through the locking block allows protrusion of the firing pin through the bolt face.