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Broken Action Slide Assembly Tube

Hello,

I am a first time Mossberg owner, and it only felt right to join this forum for information and community, especially in light of the tragedy which has befallen me (comfort me, great ones)! It is unfortunate that my first post involves an issue, but hopefully the abundant community knowledge will prevail in the face of bad luck.

The gun: 590A1 SPX
The problem: After receiving the gun last month, I anxiously went to the range to teach a few paper targets a lesson (after a sufficient cleaning and inspection of the internals, of course). First operation involved a cheap buckshot load, perhaps 15 rounds. No issues at that point, but the length of pull was a bit too long, so I felt it in my shoulder the next day. A week later, I added the Magpul M-Lok forend in lieu of the standard corncob, and the Magpul SGA stock as well. I took the gun to the range that day, and as I was cycling the action it began to feel stiffer. Suddenly when I pushed the slide forward after a shot, the action tube nut and grip slid clean off the assembly. I had to take the whole gun apart and re-tighten the nut and Magpul spacer.

Not so bad. My range buddy suggested that I put some Lock-Tite on the threads to make sure it didn't screw lose during operation. I applied some Lock-Tite Blue, thinking this would be the more gentle and non-permanent option. I tightened the nut down, not exceedingly tight, but I made it quite snug with the tool Magpul provided.

I took the gun out with me last week for some recreational shooting on a nearby mountain. After about 35 rounds of target load, the gun action began feeling stiff again. I checked the action nut but it still felt secure, so I figured maybe my lubricant was evaporating from heat. I fired a couple more shots and the nut and slide unexpectedly popped off again. But this time, the result was more gruesome:

20160304_214209.jpg
As you can see, the threading almost completely detached from the action tube, and was badly warped. I have never seen or heard of this happening to any shotgun, so I was completely shocked when I saw this, and totally bummed. Although in theory this does not prevent the gun from operating, I consider this to be a critical failure of a key component. If it happened to someone when they were in a life or death situation, this would most likely lead to the latter.

Now thankfully Mossberg's wonderful service reps offered to ship me a new component under warranty, but the question still remains: what caused this failure?

My thinking is that either the component was simply a pot metal goof (somewhat forgivable) or some kind of tolerance issue with the Magpul forearm (less forgivable). I'm not sure what the best way to diagnose the problem would be, but I sure as heck don't want it to happen again. Thoughts? Questions?
 
Is the SG new or used? How tight did you tighten the nut? One person's snug may be another's over tight. The manual doesn't state what it needs to be torqued to. The metal in that area is pretty thin so my guess it can't withstand a high amount stretching when the nut is tightened. When you get the new parts ask Mossberg what the nut needs to be tightened to. Maybe someone here can provide that info as well.
 
I'm unsure what torque the nut needs to be tightened to.

I will say that yes it was a catastrophic failure, but thankfully I can say that is the only time I have heard of it happening so that's a positive. And I'm glad they're sending you a replacement.

Hopefully this will have it lined out, and you'll get many decades of use out of it.
 
the question still remains: what caused this failure?

:confused:

Bad luck, bad machining or both.

Possibly as the nut came loose, you 'jackhammered' the nut until something gave way.

Have any weird marks on bottom of barrel? The 'stiff feeling' was probably the loose nut contacting the bottom of the heavy barrel.
 
Just got done reading this article for completely different reasons. But it is appropriate here.

Fatigue usually doesn’t play a big part in grade 8 or grade 5 fasteners since most steels are good for 2 million to 10 million cycles. Far more than you will ever winch or pull on. Here is a quick point about fastener fatigue. Almost all fastener fatigue failures are the result of improper (almost always too low) torque. Too low a torque will cause the fastener to pick up more load more often and eventually cycle it to failure. Therefore, you want to make sure you torque your fasteners to the appropriate level using a torque wrench and make sure to torque dry, clean threads.


tinelok.com
 
My guess on the threads would be perhaps the beginning of the cut was a little deeper for whatever reason and created a "weak" spot that tightening and loosening of the aftermarket nut may have brought to failure. The threads themselves look a little "gaga-ed" and end of tube looks "belled out"--what happened there? Good of Mossberg to give you a replacement.

(edit: realized this was a post on slide and not mag tube)

BTW, Welcome Aboard Brood! :)
 
Thank you all for the input. To answer/respond to your ideas...

Is the SG new or used? How tight did you tighten the nut? One person's snug may be another's over tight.

I bought it 100% new. I tightened it to be what I consider "finger tight", which is basically to the point where I can no longer comfortably tighten the nut with one hand without wrenching or forcing it dangerously. I know that's an imperfect appraisal, but I have put a lot of things together before, and I guess you just sort of know the feeling of a nut when you are going overboard with tightening. I guess I aim to turn until I feel about 8-10lbs of rotational resistance?

Bad luck, bad machining or both.

My gunsmith had the same opinion. He didn't seem to think it was a particularly well made component, although he averred the overall quality of the gun.

Possibly as the nut came loose, you 'jackhammered' the nut until something gave way.

I'd be very surprised if it came loose on its own after Loctite-ing, although I will not deny the possibility. Either way, it shouldn't be coming loose to begin with.

Have any weird marks on bottom of barrel?

No, just the regular wear marks from action movement. I was disappointed that the parkerized finish wore out so quickly on the magazine tube but I don't really care if the shotgun looks new either.

Too low a torque will cause the fastener to pick up more load more often and eventually cycle it to failure.

That is great, except that I have no idea what it means for a Mossberg slide action assembly. If I used any more torque to tighten the nut, it might have bottomed out or stripped the threading! And the gun didn't come with the nut insanely tight, so unless they designed it to fail out of the box, I can't see how I could have goofed that up.

My guess on the threads would be perhaps the beginning of the cut was a little deeper for whatever reason and created a "weak" spot that tightening and loosening of the aftermarket nut may have brought to failure.

This is quite possible, and I certainly hope this sort of defect is the rarest of the rare. As a side note, the nut itself is not aftermarket, just the one that came with the gun. The Magpul grip has two parts, the grip itself and a spacer which is supposed to provide the correct "torque" level for the shotgun model. The spacer for the 590A1 is serialized 414-0082.

The threads themselves look a little "gaga-ed" and end of tube looks "belled out"--what happened there?

God only knows, my friend. Whatever immense stress caused the steel to shear along the thread line also warped the hell out of the tube at that spot. From my shooter's perspective, it didn't feel nearly as bad as it looks when it happened, ergo my shock and disbelief when I took the gun apart to see what happened.

And thank you for the welcome! :D
 
I bought it 100% new. I tightened it to be what I consider "finger tight", which is basically to the point where I can no longer comfortably tighten the nut with one hand without wrenching or forcing it dangerously. I know that's an imperfect appraisal, but I have put a lot of things together before, and I guess you just sort of know the feeling of a nut when you are going overboard with tightening. I guess I aim to turn until I feel about 8-10lbs of rotational resistance?

Definitely not over torqued just using you fingers. Better to use a torque wrench to take the guessing out of it. I've gotten pretty good at getting nuts and bolts within a half a turn of spec before using a torque wrench. But that comes from years of "practice" in the shop.
 
Better to use a torque wrench to take the guessing out of it.

I've seen a lot of folks using those for barrel installations on AR type rifles. My only issue is that I'd need a special aftermarket forend wrench to accommodate the tool, and I'd also need to know what the "factory recommended" torque pressure is for this item.
 
It is. We've never -ever- heard of it.
I really am perhaps the unluckiest of people, in that case. Very regret ;( I'll update when my new component has arrived and throw in some sexy pics of the boom stick.
 
Definitely a defective part. When I first got my shotgun I ran it dirty and unlubed for the first quarter of its life and I got mine used. I definitely wasn't light on it. Never even came close to seeing something like that and I had a downgrip on the foregrip.
 
Not to resurrect my old dead thread, but I figured I would provide an update anyways. Mossberg eventually did get me a replacement slide action assembly, but I do not believe the Magpul forend fits correctly. If it is coming off from a lack of torque, I'm not certain how much tension I need to apply to the castle nut for safety. At any rate, I've been operating it with the original corncob for-end ever since, which fits just fine and does not seem to be at risk of ever coming loose.
 
Thanks for the update! My guess is if the nut with MagPul is coming loose, then it is not tight enough. Do you have the proper wrench? That would be helpful.

And maybe some minor cleaning up of the inside of the MagPul if needed to get the nut to seat properly? Good Luck!
 
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