I have a little problem with my JM pro that I don't understand

[robport]

I bought my JM pro for my Christmas and Birthday present. I've shot three tactical shotgun matches with it.

During the first, it locked up hard half way through the match. The match director (also a gunsmith) cleared it for me and when I took it home, I found the piston rings had been installed lined up. There was a carbon arrow on the outside of the feed tube.

Thinking I had solved it, I went to another match. It stopped feeding 3/4's of the way through the match.
Taking it apart, I found the magazine spring sprung. After I got a new one, I tried cycling rounds through it and they wouldn't cycle. After much gnashing of hands and teeth, and a lot of help from the internet, I took it further apart and found that the forward shell stop pin had broken.

After about 6 weeks waiting for parts, I went to the hardware store, cut off a 1/16" drill bit with a dremel and made a new pin. The gun operated fine for the few rounds I used to make sure that had been the problem.

Mossberg sent me a whole shell stop assembly, but the rear shell stop was too long. I scrounged enough parts from it though, to rebuild it with factory parts (using the old rear shell stop).

Last weekend, It made it all the way through a match...yeah!

Last night though, I tried to practice loading and broke another forward shell stop pin. It seemed to load just fine. I found it on unload.

Is this a common problem? If so, what can the root cause be? I was beginning to build a little trust with the shotgun until last night.

 

[tattooo]

Man...i have a 9 round JM pro & knock on wood so far that hasn't been the case with me.... ask Lazy Eyed Sniper ..... he has one also for longer than me.... best of luck

 

[LAZY EYED SNIPER]

I've had mine for over year and haven't had any issues at all. We have a few gunsmiths on the forum that may be able to offer some insight here.

@OR3GUN , have you guys seen this issue in all of your work on 930 models?

 

[OR3GUN]

Do you have a picture of the pin so I can see where the pin is breaking?

I have a couple of theories, but would need to see pictures from all sides of your bolt and carrier, as well as the removed shell stop. Some of the greatest force on the shell stop is from the back when the bolt is fully rearward. Since the problem returned with all factory parts replaced except the rear shell stop and a factory provided replacement part didn't fit, I suspect this may be where the problem lies. As you indicate the replacement rear piece of the shell stop assembly is too long, can you post pictures of the difference? Also, do you recall from your invoice if the assembly referenced both 930 & 935 fitment?

Thanks!

William

http://www.OR3GUN.com

 

[LAZY EYED SNIPER]

^ Thanks for joining in.

I'm sure your insight will be very helpful...

 

[robport]

Thanks so much for responding. I have a few pictures of the first one. I didn't keep the second pin.
These are the first parts. The second rear shell stop won't fit in what I believe is a milled slot for it in the receiver (if I'm looking at it right).

Close up of the forward shell stop and pin installed.

First rear shell stop. new one was at least a full inch longer, but I'll have to look at to tell. I'm not with the parts right now.

The first pin sheared on both ends so I couldn't even tell what it was in the forward shell stop. The second sheared on one side so that the forward shell stop was pivoting from one side of the pin and cocked. I'll try to send a picture of the one they sent me when I get home later.
Thanks again for responding!

 

[OR3GUN]

First rear shell stop. new one was at least a full inch longer, but I'll have to look at to tell. I'm not with the parts right now.

Well my first theory is probably not going to help as I was thinking you were shearing off the pin that holds the entire shell stop assembly to the receiver. But, I will share it anyway as it could still be what is happening, but not as likely.

When you get the rear shell stop in your hands, look at the very back (not visible in top picture) and at the back of the tab sticking up right next to your spring in your picture. I suspected the tolerances of the part (or perhaps your receiver's machining) was off. First, I believe you WERE sent the 935 rear shell stop, which should be a different part because of the length of bolt travel and the interaction with the carrier. What I suspected was happening was the returning bolt was hitting the back of the shell stop tab as these come within a few thousandths of contact in a gun within spec. Check the area in the center of the picture below and slowly run your bolt back and forth. You will notice the carrier comes very close to contacting the back of the tab before camming over it. Torque on the bolt handle in all directions once you reach that point and see if you can get it to contact. You would also see obvious marks from where it has done so on your bolt stop if that's what's happening, though I suspect now that it isn't.


Since it is the pin holding the front shell stop to the rear and you haven't replaced the rear (due to the wrong part being sent) I would recommend replacing the entire assembly again while incorporating the correct rear part. The holes in your rear shell stop look 'wallered out' (technical term) and are probably loose enough now to allowing ample movement to shear the pin over time. I don't know how it originally broke, but by placing the likely problematic rear shell stop back into the mix, it would lend further credibility to that specific part being out of spec somewhere since such a rare problem happened a second time.

-William

http://www.OR3GUN.com

 

[robport]

Well my first theory is probably not going to help as I was thinking you were shearing off the pin that holds the entire shell stop assembly to the receiver. But, I will share it anyway as it could still be what is happening, but not as likely.

When you get the rear shell stop in your hands, look at the very back (not visible in top picture) and at the back of the tab sticking up right next to your spring in your picture. I suspected the tolerances of the part (or perhaps your receiver's machining) was off. First, I believe you WERE sent the 935 rear shell stop, which should be a different part because of the length of bolt travel and the interaction with the carrier. What I suspected was happening was the returning bolt was hitting the back of the shell stop tab as these come within a few thousandths of contact in a gun within spec. Check the area in the center of the picture below and slowly run your bolt back and forth. You will notice the carrier comes very close to contacting the back of the tab before camming over it. Torque on the bolt handle in all directions once you reach that point and see if you can get it to contact. You would also see obvious marks from where it has done so on your bolt stop if that's what's happening, though I suspect now that it isn't.

View attachment 8121
Since it is the pin holding the front shell stop to the rear and you haven't replaced the rear (due to the wrong part being sent) I would recommend replacing the entire assembly again while incorporating the correct rear part. The holes in your rear shell stop look 'wallered out' (technical term) and are probably loose enough now to allowing ample movement to shear the pin over time. I don't know how it originally broke, but by placing the likely problematic rear shell stop back into the mix, it would lend further credibility to that specific part being out of spec somewhere since such a rare problem happened a second time.

-William

http://www.OR3GUN.com

Thanks so much!
I'll call Mossberg and try to get another rear shell stop and inspect the other parts for wear. What you said definitely makes sense.

 

[robport]

I inspected the interior and it looks like there may be interference or too much play in the shell lifter assembly. I did get another competition finished, but the next pin broke right afterwards. It was acting up a little on the last stage.

That's three pins total. I finally got hold of someone at Mossberg (that was the longest one minute in history, "estimated call wait time, 1 minute"). The guy I talked to confirmed I had been sent the wrong rear shell stop and suggested I send the shotgun back. He quoted 21 days to get it back to me. I'm a little skeptical but I'm can't afford to pay someone more than the price of the gun to fix it. Anyway, I shouldn't have to. I'm hopeful I'll get a working gun back. This is my first small step into the world of tactical shotgun and I've spent more time getting the equipment to work, rather than learning the sport. If I had known it was going to be like this, I would have never started.

 

[Mr.jetski]

Hi
Don't give up on it.
I got mine as a surprise present but had to replace the barrel out of the box cause the choke was stuck
The mag tube was replaced because it was rippled inside so the follower would jam up
Then after first match I disassembled everything and polished and blued all moving parts
The best thing I learned about this gun is to keep the outside of the mag tube carbon free ... I mean to the point of making a tool and steel wool
Then c-rums lifter and a quick loader port opening
Now 6 or 7 matches later no mechanical issues at all and runs smooth as glass

 

[robport]

I got it back from Mossberg after about 5 weeks and shot it last weekend in our biweekly event. It ran fine, so I have another 100 or so rounds on it.
What they reported that they did was a slight surprise. When I called to ask about it, they said they cleaned the piston, replaced the forward shell stop, the magazine, and the spring. I guess they threw away my new Nordic spring, but eh, as long as it works.

The only way that would have fixed it was if the magazine was hitting the shell stop, I had the wrong forward shell stop, or something like that. I have my fingers crossed. In the past, it has broken the forward shell stop pin in about 125-150 rounds. I have hope.

it ran pretty nicely Saturday. I'm almost afraid to take it apart to clean it now and I'm keeping the box it came in just in case. I'm not going to modify the loading port or lifter until I get a lot more rounds through it.

Hopefully, I can concentrate on learning to load it now. I found all 4 of the previous failures on the first few rounds of trying to learn to load it (quads) and I've tried that and it feels more solid.
If it makes it through the next event, I'll have a lot more confidence in it.

By the way, I've heard a lot of bad stuff about Mossberg customer service. They have always tried to help when I've called, so I'm happy with that.

 

[MikeT]

Joe at Mossberg's customer service needs a raise and kudos from a lot of people including me. He is the ONLY reason I have hung onto my 930. It's been ridden hard, but not as hard as my Benelli or even my Winchester 1400. So far, I have had to replace my 930's trigger group, bolt assembly, piston, pusher assembly (Several times), front fiber, front sight, and the screws on top of the receiver. The 1400 needs to be cleaned every 100 rounds as it gunks up with bulk ammo, the Benelli.... Well I just hose it out with Rem oil and keep on truckin'. After every three gun match I will clean the Benelli's barrel with a boresnake and that's it for that shotgun. My 930 now resides in my gun safe in case I ever need a failover for the Benelli. My oldest son uses the 1400 now for skeet, trap, and sporting clays.

 

[bigguygonz]

Same thing happened (twice) on my 930 SPX. Initially it would not feed from the mag tube. Would have to do some serious manipulation of the bolt release button to get a round past the shell stop, but it would double feed. Sent it back to mossberg and they replaced the entire bolt release/shell stop assembly. 50 rounds later, the same thing happened. I called mossberg and they told me they'd send me another shell stop assembly. While I was waiting, I decided to remove the shell stop. Interestingly enough, I found the retaining cotter pin was missing, though the bolt release pin was fully seated. When I got the thing out, I found this...

though I could only find two pieces, the pin clearly sheared off at the two spots on either side where the shell stop lines up with the bolt release. I sacrificed a 1/16 drill bit to make a new pin (plus an extra as backup), and the gun now at least hand cycles again. Waiting on the mossberg parts to show up so I can install the missing retaining cotter pin, before I test it firing live rounds. If the DIY solutions holds, I'll be a happy camper. I really do love this shotty, I just need it to be reliable.

I emailed Mossberg as well to let them know what I found. I've read horror stories about their customer service, but in my experience, they've been responsive, apologetic, gracious and more than helpful.