Mag tube nut loosens with firing

[Evolution]

Hi all. I have an H20 M4 with a FFT NP3 coated mag tube and I notice that the nut on the end of the mag is loosened after a firing session.

 

I did a search here and could not come up with anything, so I'm posting a new thread.

 

I'm wondering if it is because they coated the threads for the nut and the extra lubricity of the NP3 coating on the threads is allowing the nut to loosen slightly during firing.

 

Or is this a normal occurrence? Way back when I bought my 7 round tube CC did not offer their "muted finish" titanium tube, only parkerized versions, so I went with the FFT NP3 tube to match the gun.

 

Thoughts? Anyone else have this happen?

 

EDITED TO ADD; I found 2 old threads on this forum about this subject. One from 2009 and one from 2012.

 

The one from 2009 has a few people saying that it is normal and has occurred with every Benelli shotgun that they have owned.

 

http://forum.benelliusa.com/showthread.php/20404-End-cap-coming-unthreaded-when-firing

 

The one from 2012;

 

http://forum.benelliusa.com/showthread.php/29395-Severe-M4-Problems

 

A few people asked if the OP was using aftermarket hand guards and that they might be too long. The OP was using OEM guards.

 

The thing that a couple people said was that if the hand guards are a tad too long that the barrel is not fully seated into the receiver and would cause the tube nut to loosen.

 

Well, I'm using the OEM guards too and I notice with them installed before I start to tighten the nut, there is about a millimeter gap between the barrel's receiver ring and the face of the receiver which closes down under pretty decent pressure as I tighten the nut.

 

When installing the barrel assembly without the guards, the barrel's receiver ring slaps right down onto the face of the receiver without any gap and obviously no resistance tightening the nut.

 

So my question is, should there be a small gap when the guards are installed and the nut tightens the final gap of a millimeter or so between the barrel ring and receiver face? Or should the barrel ring sit down flush against the receiver face before you start tightening the nut? I just noticed looking at the barrel assembly that there is a very stout flat coil spring that seats against the front of the hand guards which causes the pressure and the millimeter gap between the barrel ring and the receiver face.

 

It seems to me that, in a way, there should be pressure on the hand guards placed on them by tightening the mag nut so they don't feel loose but, I don't know.

 

Maybe I should carefully remove some material from the front of the guards until the barrel ring just barely sets down flush on the receiver's face BEFORE the nut is tightened allowing the spring to exert only minimal pressure.

Edited June 22, 2017 by Evolution

[StrangerDanger]

Is your magazine tube loose as well, or just the magazine cap nut?

 

When tightening the magazine nut, do you feel it engaging the ratcheting teeth on the bottom edge of the magazine cap?

[Evolution]

No the mag tube has never loosened from when I installed it. I always crank the nut down as tight as it can be, but after a shooting session it has always loosened some.

 

The spring loaded cap plunger is normal in operation. I can feel the caps's teeth engaging it as I tighten it down however as it is being tightened, it feels rather mushy.

 

It's definitely because the coil spring that engages the front of the hand guards is being compressed. Should there be a millimeter or two gap between the barrel ring and the face of the receiver before you begin to tighten the cap nut?

 

Or are the hand guards a bit too long? That spring does need to exert pressure on the front of the guards, otherwise they would be loose.

[StrangerDanger]

The gap is normal until you tighten it down. I don't think that area is the problem. I'd be interested in looking at the spring loaded detent that the teeth on the magazine cap engage on. I'd also look at the teeth on the magazine cap themselves.

 

My thinking is, even if there was something wrong with the seat depth of the M4, the tightened magazine cap should not be able to unscrew by itself no matter what.

 

Another option would be to reinstall the original magazine tube to see if the problem occurs with it vs. the FFT magazine tube.

 

Can you take a picture of that detent and the magazine cap's under side? Try pressing on the detent with a flat tool to see how much tension it is providing. Look at the other end of the detent to make sure the spring didn't come out of the witness hole.

 

Other more unlikely causes are possibly a defective magazine cap. I'd look at the threads to look for any kind of weird wear.

 

If you can, take a picture of where the front of the handguards also seat. I'm wondering if they reassembled the spring correctly inside the front barrel hanger when they did the H20 plating?

[Evolution]

Thanks for the suggestions. The hand guard spring is installed correctly, it can only go in one way if it is to be held in place by the retaining slots for it and to also allow the clearance for the top of the guards. There is more spring resistance on the cap plunger pin than on the ejector pin when using my finger to depress it, and nothing amiss with the witness hole behind it.

 

The pin protrudes about 1/16" beyond the face of the support ring.

 

No, I'm thinking it's either a problem with the mag tube not being to spec in some way or the mag cap detents not cut correctly to keep it from proper engagement with the plunger pin.

 

I don't understand why FFT coats the threads on the tubes instead of leaving them in the white like CC does.

Edited June 23, 2017 by Evolution

[truckcop]

Haven't tried it on a SG magazine cap but I use Loctite anti-seize compound on my threaded pistol barrels and it keeps the thread protectors from backing off.