Remington M870 Operating Cycle

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.

Assuming the magazine is loaded and one shell is in the chamber and locked, the gun is ready to fire. The firing cycle follows.

FIRING.- With the cross bolt safety pushed to the fire position (red bands showing), the gun is fired by pulling the trigger. The top part of the trigger rotates 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.

UNLOCK.- 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 pin to the rear where it is locked and prevented from protruding through the bolt face.

EXTRACT.- 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.

EJECT.- 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.

FEEDING.- The final movement of 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 gun's action and complete the loading and locking cycles.

LOADING.- Forward movement of the fore-end will carry with it the slide, breech bolt, and 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 earner 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 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.

Maintenance

The following discussion on maintenance of the M870 shotgun will cover only action necessary for routine maintenance of the weapon. Maintenance is performed in accordance with the MRCs for this weapon.

Before any disassembly of the shotgun M870 is attempted, be sure no shells remain in the chamber or magazine.

BARREL.- To remove and clean the barrel, push the safety to the safe position. Open the action, unscrew the magazine cap, and pull the barrel from the receiver. Replace the magazine cap on the end of the magazine tube. To clean the barrel, use a cleaning rod with a lightly oiled cloth. If powder fouling remains in the barrel, use a powder solvent to scrub the bore. After using solvent, wipe the barrel clean and re-oil it very lightly. Replace the barrel by removing the magazine cap, insert the barrel in the receiver, and replace the magazine cap.

TRIGGER PLATE ASSEMBLY.- With the safety pushed to the safe position, cock the action. Tap out the front and rear trigger plate pins. Lift the rear of the trigger plate from the receiver, then slide it rearward to remove it from the gun. The trigger assembly will be cleaned as a unit by brushing with a solvent. Wipe the trigger assembly dry and re-oil it

very sparingly. When replacing the plate assembly in the gun, make sure the action bar lock enters the receiver easily and operated in position.

FORE-END ASSEMBLY UNIT.- Push the safety to the safe position. Close the action, remove the magazine cap and barrel. Reach into the bottom of the receiver and press the left shell latch inward.

Remove the fore-end by sliding it forward off the magazine tube. After the fore-end assembly has been removed from the gun, the breech bolt parts and slide may be lifted from the ends of the action bars.

NOTE

The top right edge of the slide may bind on the bottom front edge of the ejector port in the receiver. To free the slide, push downward on the front end of the bolt.

It is not necessary to disassemble the bolt for routine cleaning, Brush it with solvent to clean, then wipe dry,

Assembly of the weapon is done in reverse of disassembly. There are, however, set procedures to follow to facilitate the assembly.

Figure 8-28.-Mossberg M500 shotgun safety and action locklever.

When assembling the fore-end parts, the gun must be cocked. During this assembly, place the slide in the correct position on the ends of the double-action bar. Place the breech bolt assembly, with attached locking block assembly, over the slide on the action bars. Insert the end of the action bars into the matching grooves in the receiver. Move the fore-end slowly until contact is made with the front end of the right shell latch. Press the front right shell latch into the side of the receiver and continue moving the fore-end past this latch until contact is made with the left shell latch. Press the front of the left shell latch in to allow the fore-end assembly to pass and move freely into the receiver. Assemble the barrel to the receiver and tighten firmly with the magazine cap. This completes the assembly of the shotgun.

For further information on the Remington M870 shotgun, refer to U.S. Air Force TM TO-11W3-6-2-1.