[1KPerDay]

Hi,
A buddy of mine got this shotgun when he was a kid, his first new gun. It jammed on him. He sent it back to mossberg twice, they said it was fixed, it wasn't. He gave up. It occasionally double feeds. He has asked me to look at it for him. My first thought is a faulty shell catch. Any other ideas? What trips the shell catch to release a cartridge?

As you can see I'm not the most ideal person to work on this gun for him... but I'd like to do what I can. I have a 590 and have field stripped it and got it back together, and have a pretty good head for mechanical devices, but no specific experience with this issue. Any help would be appreciated.

[gb6491]

I have this bookmarked: http://www.thehighroad.org/showthread.php?t=185095
It might be of some help to you.
Regards,
Greg

[1KPerDay]

That will be of great help, thank you

[1KPerDay]

Well I guess I'm no gunsmith. I got it stripped and cleaned, and looked carefully at how it's working. I believe I've identified the issue, or at least one of them, but I can't see how to rectify it.

Contrary to the link above (which helped a lot) in this case it seems that interruptor is the issue. On this gun there's only one action bar, and unlike my 590, where the interruptor moves down about halfway through the slide's travel (by a machined ramp/cam on the action bar), on this one the interruptor is actuated by the movement of the elevator's downward movement, which occurs in the very last bit of the slide's rearward travel. If you pump the slide slowly you can see the interruptor sort of move a bit but not snap fully into position. The interruptor will then stay sort of partially in the way but not enough to keep another cartridge or two from sliding out, particularly if you smack the side of the receiver with your palm, while the slide's back.

I've looked closely at the cartridge stop, and it seems it has ample surface area contacting the back of the shell, and moves fully out of the way when the slide moves back. However, the timing between the stop and the interruptor seems to be off. Again, rapid, determined shucking seems to help, but then the second problem arises: chambering. The nose of the shell occasionally hangs up against the barrel. Strong enough at times to shave a bit of plastic off the edge of the shell nose. The natural reaction at this point is to return the slide fully rearward, which, if the interruptor hasn't been fully shoved into position, can allow another shell out.

I've looked closely at all the bearing surfaces of the interruptor, the stop, the elevator, and the action bar, and I see no obviously overworn parts, burrs, flat spots, etc. The bluing seems to be minimally worn. So all I can guess is that one or more of these parts is out of spec where they interact/connect. But I'm not smart enough to know which or where.

I wish I had a known-working gun to swap parts with and identify/eliminate the problem, but I don't. And I don't want to start bending stuff without knowing that whatever part is actually the culprit. Also, since the last bit of shell stop before it curves outward has to fit between the action bar and the receiver wall, my brains tell me that if I bent that outward, it may cause binding as the action bar rides over that part.

I feel bad... I hoped to be able to recify this problem for my friend. And since the issue is sporadic, I suppose that's why mossberg said it worked both times they looked at it.

[gb6491]

I have a Maverick 88 that has two slide bars, but I believe the following would apply to the single bar guns as well (the bolt slide is probably different, ie longer/wider on those guns).
As I see it:
When the slide is moved rearward, the rear of the bolt slide/carrier cams the rear of the interrupter down, raising the front of it in-between the rim of the round being fed (at the moment being held by the cartridge stop) and the rim of the round coming down the tube.
As the slide continues rearward, the slide bar will cam the cartridge stop out of the way allowing the round being fed to enter the carrier.
At this point, the next round in the tube is being held by the interrupter. Starting the slide forward will feed the round and, as the slide bar moves forward, the cartridge stop will clear it returning to the blocking position.
Following forward, the bolt slide/carrier will engage the front cam surface of the interrupter, moving it downwards and out of the way. The shell in the tube will now move forward and be retained in the tube by the cartridge stop.

From your description (and as the movement is quite distinct on my gun), it sounds like the rear cam surface of the interrupter or the corresponding surface on the bolt slide/carrier is worn/ damaged/undersized.

Then again, the single bar guns my work in an entirely different manner.
Regards,
Greg

[1KPerDay]

I think they do. My 590 works exactly as you describe. However on the single bar gun, the interrupter doesn't move until close to the very end of the slide's travel, and near as I can tell, is actuated directly by one side of the elevator (which is cammed downward by the rearward edges of the bolt slide I believe). One rail of the elevator presses on a tab on the interrupter. And this happens much more closely to the time the shell catch releases a cartridge. In fact, I think it happens AFTER the catch releases a cartridge. The hook of the interrupter doesn't move behind the rim of the first cartridge; it moves in front of the rim of the second as the first one is released. Basically in the same position fore/aft as the shell catch, If that makes sense. He's going to let me have another look at it tomorrow; I'll shoot some video if I can.

I've been told that one of the reasons they changed to the double bar was to fix complaints of double feeding. It may just be the nature of the beast. But I can't imagine it to be so. The action is buttery smooth otherwise... I wish it worked as good as it feels.

[1KPerDay]

Can someone with a single-bar 500 (or a store-brand equivalent) take a close look at the position and movement of their interrupter? From what I can tell, the new-style twin-bar actions have the interrupter positioned so the hook of the interrupter moves FORWARD of the rim of the round in the magazine (in front of the rim relative to the muzzle as the slide is moved back) whereas the older style single bar action has the hook of the interrupter positioned so the hook moves BEHIND the round in the magazine (relative to the muzzle)... but this has to happen after the first round in the mag is released by the catch.

In other words, the new style pre-positions the interrupter so it's already in front of the rim of the first round in the mag, ready to prevent the release of the second round of the mag as the first round is released by the shell catch, whereas the old style action has the shell catch release the first round and basically at the same time, the interruptor has to move quickly into place to prevent the release of the second.

Unless I'm sorely mistaken.

If this is the case, I'm not surprised that this gun has the occasional double-feed. The design seems to encourage it.

[Mike J]

1KPerDay-I have an old Mossberg 20 ga. with a single action bar. I'll try to take a look at it tomorrow-it's a little too late tonight & see what I can figure out. I'll post back.

[1KPerDay]

Thanks, I appreciate it.

[Mike J]

Okay I took out the old single action bar model 500 20 gauge & my dual action bar model 500 12 gauge. They both seem to work the same. The interrupter on both comes up when the slide is moved about halfway back & works about like you describe on your 590. It sounds as if you are right about the interrupter being the problem. FWIW I don't think it's the design but something either out of spec or out of position.

[1KPerDay]

Thank you.

[Mike J]

Sorry I couldn't be more help. Thank You-Looking at that taught me more about how my shotguns work. Never a bad thing to learn something.

[1KPerDay]

Well I fixed the double feeding issue but there's still a feeding jam sometimes and a new elevator didn't fix it.

All right, the new elevator didn't fix it.

I have a new theory; tell me what you think. I think it's the extractors.

Hear me out. As the shell is lifted, the rim has to slide along the notches in both the right and left extractors, and up the face of the bolt. As I cycle the gun slowly, I can feel resistance beginning at the point where the rim has to pass through that groove. if I press upward from below on the rear of the shell, I can feel it almost grinding along... and then it slips up into position flush with the top of the bolt face. It takes quite a bit more force than I would expect... and obviously (to me) the elevator doesn't have enough leverage to overcome it. I'm betting the extractors weren't properly relieved and there's some rough material or something along the sides and/or outside edges of that groove in the extractor.

Has anyone ever heard of this? Has anyone tried polishing the cutout in the extractors?

Any tips on removing/replacing extractors?

[1KPerDay]

HA HAAAAA! That's what it was! I believe I've fixed the gun. I compared the amount of force it took to get the rim past the extractors on an uninstalled 12 ga bolt (with 12 ga shells obviously) and the 20; the 20 was at least double the force and quite rough. The sharp edge of the right side extractor was actually shaving brass off of the side of the shell. So I carefully used a fine file and paper to radius/polish the extractors (removed the left one entirely and replaced after polishing), and polished the bolt face and THE GUN FEEDS PERFECTLY!

I can't tell you how proud I am of myself. LOL :D

[Mike J]

Glad to see you got it figured out 1K-Good Job