Mossberg JM mag tube alignment fix

[openclassterror]

Well, after having two more mossbergs in the shop this week for feeding problems, I decided to look for a simpler solution to one of the most common feed issues. One of the major problem sources on the 930 platform has been alignment between the extension tube and the primary magazine tube. Some will return to good behavior by substituting a longer follower, or one with a big bevel on the end so it can slip past the mismatch. In severe cases, we have had to resort to beveling the inside of both mag tubes, or cutting a longer follower into an hourglass shape so it can make the corner without binding. Unfortunately, that only keeps the follower from catching, and not the shell rims. A LOT of the occasional feed issues where the gun cycles without dumping a round onto the carrier are caused by either the follower or a shell catching on the mouth of the primary mag tube. It prevents the next shell from feeding until recoil bumps it loose, but that is too late to beat the shell stop. That is why sometimes the gun fires and ejects but leaves the chamber empty, yet when you retract the bolt by hand immediately after it dumps a shell onto the lifter just fine.

The source of this is the adapter nut threads not being concentric to each other. That is why some people recommend backing off the extension tub until it is loose and using the clamp to align it. If you remove the magazine end cap, spring, and follower and shine a flashlight in the receiver you can see the problem looking down the tube from the muzzle end. (obviously, make sure the gun is unloaded before pointing it at your face. I remove the trigger group to be sure). Turning either the adapter nut or the extension tube may or may not improve the mismatch. The first photo is an out-of-the-box JM Pro, showing what I mean. If your shotgun looks like this, then EVERY TIME you take it apart for cleaning you run the risk of it lining up like this when you re-assemble it. That is why some people have said their shotgun ran fine until they cleaned it, then started jamming. Then they take it apart to look for problems, every thing seems normal, re-assemble, and viola, it works again! So- Just for the heck of it, since we were running extension nuts for the Stoeger M3000 on the CNC lathe, I decided to make one with the correct thread for the Mossberg 930. Our manufacturing method results in thread concentricity within .001 total runout. I installed one with no other changes on another 930, and the result is picture number 2. So, all this time, the nut has been the alignment problem! The factory follower is still ridiculously short and should be replaced, but both the Nordic low-drag follower and ours slid right past without hanging up. SO, how many of you are gonna look down YOUR magazine tube after work

[mpom]

Any plans for production and what will the damage be?

[openclassterror]

Production would be dependent on interest. I don't know how many of the factory nuts are this bad. If there are a lot, then we could run a batch for sure. If we decide to, details would be on our Vendor page, rather than here on the Tech forum.

[usmc1094]

I had a feeding problem with my 930 JM, but after I put on a RCI extension it all went away. It is a pricey way to fix it but it has been perfect ever since.

I'm sure if Tom would make a fix it would be top notch!

Edited October 23, 2014 by usmc1094

[HeyChris]

I've never had a problem with my JM Pro feeding.. but isn't that a Nordic nut you are talking about. Are you saying the Nordic adapter nuts aren't concentric?

[openclassterror]

I am not saying Nordic nuts are not concentric. I am saying that the nuts Mossberg uses, wherever they come from, are at least a fairly high percentage of the time, not concentric. Is it a known fact that Nordic makes the JM Pro nut? I know the extension tube is not a standard Nordic tube. All Nordic MXT tubes use the same thread on both ends and are reversible. The Mossberg tube uses a smaller thread for the cap end than they do for the nut end. The Nut also uses a different finger groove pattern than Nordic typically uses. That doesn't mean they aren't sub-contracting to Nordic, but at the very least they are not the standard MXT series components.

[hysupra]

They are not the MXT series they are the older version.

[HeyChris]

I am not saying Nordic nuts are not concentric. I am saying that the nuts Mossberg uses, wherever they come from, are at least a fairly high percentage of the time, not concentric. Is it a known fact that Nordic makes the JM Pro nut? I know the extension tube is not a standard Nordic tube. All Nordic MXT tubes use the same thread on both ends and are reversible. The Mossberg tube uses a smaller thread for the cap end than they do for the nut end. The Nut also uses a different finger groove pattern than Nordic typically uses. That doesn't mean they aren't sub-contracting to Nordic, but at the very least they are not the standard MXT series components.

On my JM Pro, it came with a Nordic nut and Nordic +5 tube with end cap.

I just ordered a +8 Nordic MXT Modular tube and it uses the exact same nut. Same finger grooves and everything.. which is why I was asking about it.

[openclassterror]

Hmmm that is interesting. They might want to look at their fixturing if that is the case. Does the new nut/ tube assembly you got from Nordic have the same type of mismatch as the first picture?

[dhunt91]

Mine does this during dry loading practice. Never had it happen in a match tho.

[HeyChris]

Hmmm that is interesting. They might want to look at their fixturing if that is the case. Does the new nut/ tube assembly you got from Nordic have the same type of mismatch as the first picture?

I looked at both the old and new assembly and on my shotgun, neither show the mismatch that appear in your picture. Unfortunately I couldn't get a good picture with my phone.

Both Nordic nuts fit snug to the shell tube on the shotgun. I then make sure the extended tube is tight and back off one turn and secure barrel clamp. When I look down the tube, alignment is all good. I'm also using the stock follower, which I've not had a problem with. Only modifications I made was to polish the outside of the follower.

It has been my understanding the purpose of backing off the extended tube before tightening the barrel clamp, was to allow play and eliminate pressure on the barrel. Nordic provides that instruction with the extension tube.

Here are the Nordic nuts I have, top is the new +8, bottom was the original +5 that came with the shotgun. Same nut, I can swap the parts back and forth.

I'm not challenging your analysis, I was posting to show that on mine at least.. these are Nordic nuts. It certainly doesn't hurt if folks are having issues to make sure they are getting and "can get" correct alignment in the tube.

Edited October 23, 2014 by HeyChris

[openclassterror]

You are correct about backing the tube off a turn to remove stress on the barrel. I am referring to the practice of continuing to turn the tube until the mismatch improves and clamping at that location to improve alignment. It does not surprise me that if your alignment is good that the factory follower would not catch. I prefer a longer follower because it is less likely to bind, but in a perfect world where everything lines up it would not be necessary.

Those are indeed the same nuts that are coming in on the shotguns I keep working on. Now understand that since I am a Gunsmith, it stands to reason that I see an abnormally high percentage of bad ones, because people don't bring in the ones that run. If some have good alignment (as yours does), that would explain why some run great from day one, and others have issues right out of the box.

Out of curiosity, did you look down the tube while rotating the nut on the gun to see if it goes wee-waa with the nut backed off? It may be that your nut happens to line up perfectly when tight against the fore end but not at a half turn out. The thread timing might affect why some line up and others not so much. Or else, some of the Nordic nuts are true, and others are machined off-center.

[HeyChris]

Out of curiosity, did you look down the tube while rotating the nut on the gun to see if it goes wee-waa with the nut backed off? It may be that your nut happens to line up perfectly when tight against the fore end but not at a half turn out. The thread timing might affect why some line up and others not so much. Or else, some of the Nordic nuts are true, and others are machined off-center.

I didn't check, but I'll try and make note the next time I give it a cleaning.

[rtp]

I am not saying Nordic nuts are not concentric. I am saying that the nuts Mossberg uses, wherever they come from, are at least a fairly high percentage of the time, not concentric. Is it a known fact that Nordic makes the JM Pro nut? I know the extension tube is not a standard Nordic tube. All Nordic MXT tubes use the same thread on both ends and are reversible. The Mossberg tube uses a smaller thread for the cap end than they do for the nut end. The Nut also uses a different finger groove pattern than Nordic typically uses. That doesn't mean they aren't sub-contracting to Nordic, but at the very least they are not the standard MXT series components.

Different timeline. The MXT is the 'new' Nordic tube setup, but AFAIK, the JM Pros all had Nordic mags/caps/nuts on them as shipped, while the non-JM 93-s had Choates.

This is why some of us kept the original JM mag tubes and simply added a L/XL/XXL Nordic end cap..onto our Nordic tubes.

[rtp]

Here are the Nordic nuts I have, top is the new +8, bottom was the original +5 that came with the shotgun. Same nut, I can swap the parts back and forth.

I'm not challenging your analysis, I was posting to show that on mine at least.. these are Nordic nuts. It certainly doesn't hurt if folks are having issues to make sure they are getting and "can get" correct alignment in the tube.

Wait - does that imply you can use a 'new style' MXT tube on the JM with the original (Nordic) nut? I could have sworn I read you had to buy 'everything' to go to the newer MXT style tube.. ???

Out of curiosity, did you look down the tube while rotating the nut on the gun to see if it goes wee-waa with the nut backed off?

Too technical for me; I can never remember the difference between wee-waa, wee-wee, and fubar.

I'll take a look at my JM's tube this weekend, while rotating the tube. I recall doing it when I first got the gun and it 'seemed' like I got a 'good one,' meaning I didn't have to do any beveling to 'help' the tubes mate up.

I do intentionally run a longer RCI follower, but otherwise nothing magic - OE Nordic tube + Roulette Tactical +2 or Nordic XXL end cap, Nordic mag spring, polished internally on the tube.

[bikerburgess]

Yes you can put a MXT tube in the stock JM nut you will need a new end cap that is different

[mpom]

Checked and sure enough, tubes were not concentric. Not surprised as the follower stops at the junction, then continues after a short pause. I beveled the forward end of the fixed mag tube, with no improvement. After checking today, I took the Nordic factory extension and ground off around a 1/16" from the end screwed into the nut, thinking that long bevel on the inside of the section inside the nut would allow a shell rim to go off center and hang up. I tested the problem before the modification to the tube with a shotgun cleaning rod minus any attachment, and the end of the rod would hang up on the protruding part of the tube that acted like a shelf. After the mod, the cleaning rod had a harder time finding a shelf to hang up on. Not perfect but definitely better. No doubt Tom is on to something and I would be interested in a concentric nut at a reasonable price. More for insurance than to fix a malfunction.

[Nickoli]

I never had the hang up like that but I would hear the follower hit that area when ever I dropped it in. I ended up going to a RCI extension and never had an issue, since then I have switched back to Nordic with the new MXT tube and coupler and I have not had an issue in thousands of rounds. I have also cleaned up the tube on the receiver just to make sure.

[TitoR]

Ok, I have been experiencing some feeding issues and was curious about this post. So, I decided to check my extension alignment. No matter how I turn the extension or the nut to align the tube and extension, the stock follower hangs in the transition. I even removed the nut and mated the extension and tube by hand, with the same results.

So, I measured the inside diameter of the extension and the shotgun magazine tube. Extension is .935 and magazine is .915, so there is a .020 step between the two. So, the follower will hand up feeding into the chamber, since it is going into a reduction in diameter, but will freely move in the loading direction. Openclassterror, are these measurements consistent with the shotguns you've worked on?

Also, the extension end that mates with the shotgun magazine is beveled allowing it to wrap around the magazine tube, so I believe the concentricity issue may also arise if the bevel of the extension is not concentric, allowing the mag tube to sit offset, even if the nut is concentric.

So, I would consider a total fix one that takes into consideration the difference in inside diameters and the concentricity of the barrel nut. So I am going to start by beveling the inside of the magazine tube to create a smoother transition from the extension. I'll post if this has any effects on the feeding of my shotgun.

[openclassterror]

Yes there is a diameter mismatch on all of them. As I stated in the opening post, beveling the mouth of both mag tubes was part of our usual fix for the sticking follower issue. Also, the factory follower is VERY short, which allows is to tip in the larger diameter tube at the junction. There is essentially NO chamfer on the end of the factory follower. Substituting a longer follower with a small chamfer on the leading edge keeps it from binding. If the tubes are concentric, any amount of chamfer greater than the diameter mismatch will prevent catching. If the tubes are offset, the chamfer must be larger than the mismatch on the worst side.