To replace the trigger assembly, tilt the trigger assembly slightly towards its angled lever. This is the slide lock lever. Ease the slide lock lever into the receiver, and then press the trigger assembly into place. Line the holes up and press the trigger pins through the receiver.
The Mossberg Model 500
The Mossberg has many design characteristics similar to the old High Standard pumps. High Standard went out of business in the early 1970s but many of their shotgun still serve faithfully.
The big advantage that Mossberg had over High Standard was its removable barrel (yes, High Standard was making shotguns thal did not have a removable barrel, even into the 1960s!).
A broken safety on the Mossberg 500. On top, it can get in the way of obstacles and become cracked or broken. Replacement is easy.
While desirable, the removable barrel is not the remarkable thing about the Mossberg design. It has four things that make it an exemplary shotgun; the safety, the shell stops, the receiver itself and the shell lifter. The safely is a button on the top center rear of the receiver. For decades the Mossberg was unique in this. While other shotguns had a button in the triggerguard that had to be replaced for left-handed shooters, the Mossberg didn't. The shell stops in the Mossberg design are not staked into the receiver. Staking the shell stops would be a spectacularly bad idea because the receiver is aluminum. Instead of being made from a steel forging, the Moss-berg's aluminum receiver is easier to machine, lighter and rust-free. The shell lifter rests tight against the bottom of the bolt carrier. It only drops down into the path of a shell from the magazine when it is needed. On other shotguns, you can “double feed” a shell. If, when you press the shell into the magazine, you do not press it far enough for the shell stops to catch it, the shell can pop back into the receiver. Trapped between the carrier and the lifter, the shell wedges the action tight. In a Mossberg, if you do not press the shell in far enough for the stops to catch it, it will drop back out on the ground. Yes, it is an annoyance to pick it up, but nothing like the headache of correcting a double feed.
The Mossberg lifter stays tight to the bolt when closed, and you cannot double-feed a Mossberg. If the shell is not caught by the shell stops, it gets flung to the ground.
When inserting a shell, push your finger or thumb into the tube. This ensures the shell is caught by the shell stops.
To disassemble the Mossberg, make sure it is unloaded. Open the action. Turn the knurled nut on the front of the barrel extension. The nut will stay on when the barrel it is removed. Once the nut is unscrewed, pull the barrel off. On the receiver there appear to be two pins. There is only one, and it is the smaller one farther back from the ejection port. Drift this pin out. The trigger mechanism is held in place at the front by two shelves that fit into recesses in the receiver. Pivot the rear of the trigger assembly down and out of the receiver. The shell stops will fall out on their own. Slide the forearm back until the rectangular plate under the bolt lines up with the wide slot cut through the bottomof the receiver. Lift the connector plate out, and push the bolt forward out of the receiver. The shell lifter is a tuning fork-like piece of spring steel. Unlike the 870 it is not attached to the trigger assembly, and is powered by the bolt. It pivots on two circular bosses that can be seen above the trigger assembly retaining pin, this is what appears to be another crosspin, but isn't. To remove the lifter, use a small screwdriver to pry the rear of the lifter out of the circular holes that have been drilled through the receiver. Scrub the parts. With all of its advantages, you have to figure the Mossberg has some drawback. One small one is the magazine tube. Because the barrel retaining nut screws into the end of the magazine tube, you can't remove the spring and follower that way. Instead you must unscrew the magazine tube. Clamp your receiver in a padded vise. If you are strong you can unscrew it by hand. Otherwise, use a strap wrench or a padded pair of curved pliers to unscrew the tube. To reassemble almost requires three hands. The follower stop shoulder is machined into the receiver, and as you attempt to screw the tube in the follower and spring will push back. Use your right hand (for right-handed 'smiths) to turn the tube, while with your left hand you press the follower back into the tube. Once you have the tube turned all the way into the receiver you can let go of the follower, and give the tube its final tightening turn. You needn't tighten it more than hand tight. Once the barrel is in place the magazine tube isn't going anywhere.
The simplicity and durability of the Mossberg 500 is obvious. No small, easily bent or broken parts, and nothing to keep you from seeing (or feeling) if there is a shell in the magazine.
What looks like a disassembly pin on the right side is the pivot rod for the right-hand shell stop.
To reassemble the receiver, turn the lifter until the hooked end is towards the safety, and the points of the hooks are towards the bottom of the receiver. Squeeze the rear of the lifter enough to slide it into the receiver. Press it down until the two circular bosses snap into their holes. Push the bolt into the receiver, and push the forearm back until the bolt and action bars are positioned in the rectangular slot. Place the slide plate down on the bolt, and engaging the action bars. If you do not have it in place, the forearm won't go forward, or won't take the plate and bolt with it. Replace the barrel. Run the slide forward. Turn the receiver upside down on your bench or in a padded vise. Place the shell stops in their slots, with the tabs in the space between the operating rods and the receiver. Each stop is designed to fit only in its appropriate slot. Take the trigger assembly and catch the shelf on its front into the slots in the receiver. Pivot the rear into the receiver and press the crosspin through.
The Mossberg shell stop are not staked in place, and do not have to be timed if replaced.
When the bolt is back, the rear legs of the lifter cam against the bolt slide and the front is forced down.
The action rails are simple steel bars.
When the bolt goes forward, the lifter is cammed up, raising a shell into the bolt's path, and clearing the magazine tube for further loading
The bolt slide rests in the action bars.