MP38 Loose Barrel Nut

After a 5 month wait, I just received my C&R MP38. I'm looking forward to getting it out to the range, however I noticed my barrel nut was only hand tightened.

Is re-tightening this as much as possible by hand sufficient for firing or does the barrel nut need to be really cranked down tight with a wrench? I don't want to risk damaging anything with it coming loose under fire!

Is the flat washer in between the nut and trunnion? Those are there so you can tighten down and the little protrusions on the spring fit into the little cutouts on your nut to prevent it from spinning off with out using a wrench. I hand tighten mine and then give it a little turn with the wrench just past where I can't hand tighten anymore.

The thin washer is there but it is loose and rattles around even with the barrel nut fully tightened by hand. So I don't think it is helping much.

The barrel is mismatched. Could this be a tolerance issues?

Any other ideas? I'm hoping to test it out at a buddy's ranch tomorrow.

With the barrel guard in place on the bottom of the barrel the nut won't be able to come off anyways. Sounds like the mismatched barrel might be the culprit. Mine has the original barrel, never been off. My nut won't budge. Tried to remove it to hammer some dents out of the receiver and snapped my breaker bar trying to loosen the nut. Needless to say it never did come off. I'd just snug it up and just keep an eye on it. As I said, as long as the barrel guard is in place the nut cannot come off anyways.

Good point on the barrel rest. It is likely applying enough tension to prevent it from randomly turning off. That said, the rest is not designed to come off in order to take the barrel off. At least none of the ones I have do, you simply turn the nut and the entire assembly moves forward as it comes loose. 8x57mauser - I suspect your nut is just frozen in place.

Meridius - You might try taking it apart and cleaning out real good inside the nut and around the barrel. It may be that there is some debris in there or a nick in the internal threads or somewhere else causing interference. I have had that happen before. It doesn't take much.

KSH67 said:

Meridius - You might try taking it apart and cleaning out real good inside the nut and around the barrel. It may be that there is some debris in there or a nick in the internal threads or somewhere else causing interference. I have had that happen before. It doesn't take much.

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This - I use an old toothbrush and gently brush the threads in the correct direction, sometimes a little oil, and it is amazing the dirt that comes out of the thread - MP44 barrel nuts always seem to be full of gunk.
One tiny piece of grit will stop the piece from tightening.

Great looking MP38 - well done!

8x57mauser said:

With the barrel guard in place on the bottom of the barrel the nut won't be able to come off anyways. Sounds like the mismatched barrel might be the culprit. Mine has the original barrel, never been off. My nut won't budge. Tried to remove it to hammer some dents out of the receiver and snapped my breaker bar trying to loosen the nut. Needless to say it never did come off. I'd just snug it up and just keep an eye on it. As I said, as long as the barrel guard is in place the nut cannot come off anyways.

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With regards to your Barrel Nut, did you try heat? I've had H bands lightly tap on and then refuse to come off almost to the point of damaging the band... A little butane torch from H Freight does it every time...

And great looking MP Meridius!

Thanks for the advice everyone. I half assed tried to clean the threads out a few days ago but I will try again to make sure they really are clean. I did try putting in another spare barrel to see what would happen and I ran into the same problem.

Meridius said:

The thin washer is there but it is loose and rattles around even with the barrel nut fully tightened by hand. So I don't think it is helping much.

The barrel is mismatched. Could this be a tolerance issues?

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Yes it is a tolerance issue created by debris in the threads and/or the tolerances of the contact points between the barrel and trunion. That's why those parts are numbered The barrel nut should snug up tight to the trunion and the thin washer should keep the nut from turning because the washer was originally staked to the notches in the barrel nut and sling ring. If the tolerances won't allow the nut to snug down on the barrel washer then there is the issue, though you could tighten it down enough, perhaps adding a little BLUE! loctite, to keep the nut on. Personally, I'd add an extra washer. Those can be found in the $15 to 25 range, or if you are handy with a file and dremel I bet you may be able to find something similar dirt cheap at a supply house and make it work. Perhaps even an o-ring in heat resistant rubber or resin would work to keep everything tight.

Note that in all of this the important issue is headspace and the area where the chamber/feed ramp fits through the trunion on the inside of the receiver where the bolt slams against the cartridge head and the extractor grips to start the ejection process. That area, and ejector angle, and feed angle, are critical. Let us know how it works out Meridius!
Cheers,
Hambone

Meridius said:

Thanks for the advice everyone. I half assed tried to clean the threads out a few days ago but I will try again to make sure they really are clean. I did try putting in another spare barrel to see what would happen and I ran into the same problem.

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Could be sling ring thickness / tolerance too. Is it original? Did you try a different one of those? I've found MP.40s to be more out of spec / non interchangeable re parts than any weapon system I'm hands on familiar with. They can sometimes take some tuning. However, they are so simple that a problem can invariably be ID'd and corrected pretty easily. Once it's all squared away they are robust and reliable. I'd trust my life to a proper one.

EDIT: For example:
http://www.ebay.com/itm/MP38-MP40-W...256?pt=LH_DefaultDomain_0&hash=item3cf5578c68