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Loose lock nut

ancrox

.22LR
44597218c37615258e74791efd85aa08.jpg
Put about 250 rounds through my MMR Hunter the other day. Next day while cleaning it, I noticed my handguard was slightly loose. After I checked it, turns out it was the lock nut instead.
I hand tightened it for now.
I need to know what happened and how to fix it. I'm mechanically inclined so I don't want to go to Smith.
Thanks for any info.

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Try tightening it with a strap wrench? I would avoid Loctite because that could make removal problematic... Maybe remove it all the way and make sure the threads are clean then re-tighten it?

UPDATE: disregard the above if it doesn't apply--I was referring to a full-float handguard... Like LES says below--be careful of the gas tube...
 
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Not sure how that particular barrel nut works, but be mindful of the gas tube.

The gas tube likely runs through a hole in the nut and tightening with the tube installed could bend or otherwise damage it...
 
Is it me or do i not see the gas tube? Or, even a way for the gas tube to make it to the upper?
 
So I guess I need to remove the handguard and remove the gas tube first?

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It would look like to me that you have an inner barrel nut (that the gas tube goes through)--that is the thing with all the holes. THAT part should not be loose at all!. Then you have a full-float handguard that screws onto the barrel nut and a "lock" nut that screws against the handguard to keep it in the proper orientation so your swivels remain at 6-o'clock. So yes, a small strap wrench on the lock nut to help tighten against the handguard. Strap and "generic" barrel nut wrenches shown below. You do not need the barrel nut wrench, I'm just showing it so have an idea of what one looks like--there are many different styles...

Nice looking gun, BTW... :)

strapwrench.jpg barnutwrench.jpg
 
A couple more pics:

First shows what it looks like under the handguard. That is a short pistol barrel and the gas tube wraps around to increase effective length the gas has to travel. Yours would be straight. You can also see the gas block that clamps over the gas port in the barrel. Some gas from a fired round is ported off the barrel, into the block, into the tube, then into the gas "key" on the bolt, pushing the bolt rearward.

Second pic is showing what you need to do...

nuttube.jpg 8larue7.jpg
 
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