Dealing with double feeds , late feeds, and no feeds of the shell/cartridge from the magazine of the Mossberg 500 and other shotguns is a matter of controling the shell/cartridge stop and the interruptor. (info below assumes a clean gun)
On the Mossberg 500 series guns the stop is on the inside left panel of the receiver and the interruptor is on the right side internal panel. They are held in place by the trigger housing and are easily removed (or fall out) when the housing is removed from the receiver .
The cartridge stop is actuated in and out by a small tab protruding upward near the front of the stop (just back from the front curve) as it interacts with the left slide action bar . (tappered ramps meet during the feed cycle and pull the stop to the left releasing the shell from the magazine. When the bolt is forward, the shell stop is allowed to be at rest and hold the shell head to prevent shells from coming out of the magazine tube.
The cartridge interruptor is actuated up and down on it's pivot with its interaction with the bolt slide. The front of this interruptor moves downward during the feed cycle to prevent the second shell in the magazine from coming out . (double feeding). When the bolt is forward the head of the interuptor is held out of the path of the shell head in the magazine allowing it to push up against the shell stop where it is held in place. (read above)
The most frequent adjustment needed in this sytem is with the cartridge stop. If not adjusted properly it will allow the shell head to pass out of the magazine at the incorrect time, or it will hold the shell into the magazine when it is suppose to release it into the action. Since there is some variation allowed in the diameter of the shell base in catridge manufacturing there are times that changing brands, or lots ,of shells can correct feed problems. A much better way however is to correctly adjust the stop so that it will reliably feed all brands having some tolerance built into the gun by adjustment.
To adjust the shell stop you should first observe its range of movement during operation. You can use dummy rounds, feeding them with the gun held upside down to observe how the end of the stop mates with the base of the shell. With the bolt forward there should be sufficeint engagement to hold the shells into the magazine.(even when wiggled slightly by hand the shell head should not jump past the tip of the stop).
If this is not the case, the front half of the stop can be adjusted (bent) to increase the engagement. Do this by removing the stop from the gun , and clamp the rear half in a vise, then bend the front in the direction of the curve in the front tip of the stop. Do not overadjust, it takes only a small amount to make a big difference. If overadjusted the ramp ingagement of the stop and its actuator ramp on the action slide bar will not mate properly. This can cause malfunctions and can damage the tips of the ramps.
If the shell stop is holding the shell in the magazine with good engagement, but will not release them during the feed cycle, then you must adjust the tip of the stop by removing just enough material from it to give clearance for the shell head to pass while the stop is pulled out of the way. Making sure that you have proper engagement of the ramps to provide the full movement of the the stops tip out of the way is important before you remove material from the tip. Again, this doesn't generaly take a lot of adjustment so remove only a small amount of material at a time using a file or dremel tool.
If you study the operation well before adjusting you should be able to tune this stop to the middle of it's proper travel and take away the possiblity of small variations in the diameter of the shells base causing a problem.
With the shell stop working properly there is seldom a problem with the interruptor. The tip of the interuptor has a less critical range of motion and is controled well by the pivot method. What the interruptor does is swing into position during the feed cycle before the first shell clears the magazine. By being in a position in front of the magazine opening at this time it prevents the second shell from being released during the action cycle. If it does not hold the second round from coming out of the magazine the result is a double feed jam. If it does not allow the first shell in the magazine past it to rest onto the shell stop, you will have complete failure to feed out of the magazine. Adjustments can be made to the tip of this stop by bending it in the direction needed to prevent the problems mentioned - it's just that this is a seldom seen problem.
So that's about it folks - I hope this helps somebody and questions/corrections are welcomed. Keep in mind also that these parts are not all that expensive and having a second set isn't a bad idea. Typically though, they don't wear out so to speak - they generaly are out of adjustment rather than worn out. A little tweak can make a big difference in reliability , and although this info is specific to the Mossberg, it can serve as a guide for many other brands of guns as well.
This is a great question. The interrupter, along with the shell stop, are confusing parts. Old Mossys reply is great. We will explain these parts a little more with moving pitchers! Todays model is the handy-dandy 590 Mariner. We've swapped in matte parkerized parts for contrast.
The task of the cartridge interrupter is to keep shells in the tube when the shell stop is out of the way. Plain and simple. Without the interrupter the magazine tube will spit up all the shells at once as we'll see. The interrupter is blocked by the locking block and rotates on a pivot. This is why it cannot be staked in place like an 870.
awesome ! thanks for the great replies. I now fully understand clearly the function of the interrupter.
I looked for hours and could find basically nothing. next time I'm not gonna waste time and just ask here.
Here's something from a gunsmith course on the 500 that you might like.
Note #1. How many cartridge stops does the Mossberg 500 have?
a) 1 b) 2
Note #2. What side of the gun is the primary cartridge stop located? a) Left side
b) Right side
Note #3. What does the secondary cartridge (cartridge interrupter) stop do?
a) Holds thefirst cartridge to be chambered from the magazine tube in place b) Holds the second cartridge to be chambered inside of the magazine tube until the first cartridge is chambered
c) There is no secondary cartridge stop (cartridge interrupter)
Note #4. What makes the secondary cartridge stop (cartridge interrupter) move up and down?
a) There is no secondary cartridge stop (cartridge intemipter)
b) The cam on the action bar c) The bolt slide (and bolt)
d) The follower
Note #5. The primary cartridge stop is moved in and out by a) the action bar
b) the bolt slide
c) the follower
First post on here. This thread seems appropriate but maybe I should make a new one, just let me know and I will. Ok, here is my current situation that I am looking for feed back on:
Purchased a new 500A 18" barrel years back. It had a feeding issue (shells would not come out of mag tube when racked) most likely the cartridge stop but not known. I sent it back to mossberg and they repaired it (not sure what they did but it worked ever since).
The other day I was researching on the web how to make the pump action smoother. Was not able to align the pump handle insert rods well as suggested but I did sand the internal moving part surfaces that rub each other with 500 grit wet sand paper. This removed some of the parkerised coating but oh well. I figured it was a home defense gun so that would be fine. In this case I will trade smoother action for less corrosion protection of these parts. Not sure if this was the "right thing to do". Anyway just purchased a new camo 500 12 ga 28" ported ribbed barrel with chokes for duck hunting (came with 18" barrel) for $390 on sale at big5. After days of agonizing web research (camo versus black gun for duck hunting) I determined I should have just bought the black version that went on sale at big5 for $309 the next week.
Anyway get home with the new camo gun and it has the same problem. Quick field strip cleaning same issue. Removed the dowl rod same issue. Different shells same issue. The cartridge stop was not allowing any shell out of the tube when racking slide. Did all kinds of testing and was about to send the gun to mossberg for repair when I decide to swap cartridge stops on the two guns. Now they both work very well. The home defense black 18" gun just has a slight additional catch from the cartridge stop when loading shells into the mag tube. It takes a little extra push to get them all the way in there. I notice the new cartidge stop has a slight raised cylinder feature on the receiver facing side that the old on does not and found that very interesting. Sorry for the long winded post but here are my questions:
1) Does you guys clean a new 500 thoroughly before shooting it? Or just field strip and a quick wipe down and oil, shoot (test), then deep cleaning?
2) I noticed a redish brown coating on the internal 500 parts. Is this rust from long term storage or a rust inhibitor for long term storage?
3) For duck hunting in a salt marsh will the internal parts treatment I did for action smoothing (removing the parkerized coating wiht 500 grit wet sanding) with this cause more corrosion that if I left the coating on? I am trying to determine if I should repeat this treatment on my new 500 but fear losing the corrosion protection for salt marsh hunting.
4) Is swapping these parts OK? I will post pictures of the parts side by side ASAP
Noticed I sanded one down for smoother action and took some of the parkerized coating off. It smoothed the action but may have cost me corrosion protection. I was reading up on the internet how to smooth the action and it was saying rack it a hunred thousand times or do this. I have no idea whats best but it did feel like more than a few rough machining/stamping burs and edges we removed.
Notice that raised cylinder on the newer one. Wonder why that is new? These were only purchase 4 years apart. One last week and one four years ago.