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hondo

DPMS Low upper barrel nut aligns but doesn't!

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DPMS .308 low build. The Mentium USA free float handguard barrel nut fits and is aligned but the gas tube can't be inserted. Imagine a clock face, 12-3-6-9, at the 12 position, under the flat top, the gas tube hole alignment rib is blocking insertion of a pin/gas tube. At the 3-6-9 position a smaller tube inserts and doesn't snag when inserted. So I placed a weapon light into the upper and it appears to be at least 0.060" of an obstruction. Once again, ALL of the alignment issue is at that one rib of the uppers gas tube hole, vertical position when holding the firearm. The barrel nut looks like a DPMS barrel nut with 22 holes. A PRI DPMS/KAC barrel nut wrench fits the barrel nut and was used to torque the barrel nut. 

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can you snap a pic of the barrel nut? there are several styles and a pic will ensure we know exactly what you are talking about

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Yep, pics will help.  I think I know what you're talking about already, and have a solution - if it's what I think it is.

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I've found references to at least three DPMS pattern barrel nut variations, 0.86" radius/1.720" bolt circle diameter, 0.845" radius/1.690" bolt circle diameter, and EGW 0.869" radius/1.738" bolt circle diameter. Just for fun I torqued up an EGW  barrel nut and the interference was worse at the 12 o'clock position. I could of course use a barrel nut such as a JBL where the gas tube goes over the barrel nut rather than through the barrel nut but I'd like to use one of the free float tubes I have on hand. Interesting that both of the above mentioned barrel nuts ended up at 90ft.lbs of torque to line up. I'm going to temporarily mount the gas block with tube to check if the uppers gas tube "cloverleaf" is in spec.

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So you are saying the gas tube fits through the holes in barrel nut by itself...and it fits through the somewhat X shaped hole in the upper by istself, but when the barrel nut is on the upper than the gas tube wont go through? Is this accurate?

If this is the case, you either need to crank it a C hair in the required direction to get it to clear that little rib in the upper hole if its off left or right, or the barrel nut is machined a tiny bit off if its off up or down. Ive never heard of that brand so that would be my guess. That hole in the barrel nut isnt super important and you could get away with running a drill bit through there if you need to. 

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@blue109, it sounds like the radius from the barrel to the gas tube hole in the barrel nut is off. He's saying at 12 o'clock, inside the gas tube hole in the barrel nut, the top of the X or cloverleaf in the upper receiver is blocking the tube from being inserted.

@hondo, am I reading you correctly?

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Yes, the longer radius EGW barrel nut made the mismatch worse. An 0.845" radius barrel nut would probably be ideal. I'm thinking about ordering a spare barrel nut and drilling out the offending hole.

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Pics, man.  Pics.   :thumbup:  Don't know what that was that you attached previously, but it wasn't a picture format that can be viewed on a computer.

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It opens for me but here is a link, https://yhm.net/accessories/forearms/handguard-accessories.html scroll down to part YHM-9483. I know this part is for an AR-15 but that is what the Mentium USA barrel nut looks like. I believe the problem is that although both the Mentium USA and EGW barrel nuts are DPMS 1 7/16"x16tpi, the gas tube radius holes are Armalite radius of 0.860", 0.869" in the case of the EGW instead of DPMS radius of 0.845".

In the case of the EGW free float tube it was inexpensive but exactly what I wanted, a plain tube. The Mentium USA was on sale, stated to fit a DPMS Low upper and the rib matches well but without the gas tube installed due to the barrel nuts Armalite gas tube hole radius.

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11 hours ago, hondo said:

It opens for me

Is that because you're on a phone, and it's in the format of your phone's software?...  It's not showing here, on a PC.

11 hours ago, hondo said:

but here is a link, https://yhm.net/accessories/forearms/handguard-accessories.html scroll down to part YHM-9483.

That's not a link, but here's how you do this - this is the link to what you described:

https://yhm.net/barrel-nut.html

Here's the barrel nut in that link, from YHM:

yhm-9483_3.jpg

So, the exact issue is that it thread onto your barrel, you can insert the gas tube - and then the "port" in that barrel nut is on a different plane than the hole in the upper receiver?

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Yes, the barrel nut "port" that aligns with the uppers "cloverleaf" is too high vertically when torqued, i.e. the 12 o'clock position. I need a brand name forearm for a DPMS Low upper that has a DPMS gas port 0.845" radius/1.690" bolt circle diameter barrel nut rather than the two previously mentioned that thread-on and torque fine but are Armalite gas port 0.860" radius bolt circle diameter. In other words a smaller radius/bolt circle diameter would lower the barrel nut gas port holes. I'm thinking about drilling out the offending port 1/32" larger which will lower the bottom of the "port" but only if I can get a spare barrel nut. I've e-mailed both Mentium USA and EGW to see if they would sell their barrel nuts by itself because if I scrap the one that came with the forearm(s) then I've gained nothing. I found a thread on this forum that referenced a gas tube "rubbing/laying on" a barrel nut without adequate clearance to enter an upper but the thread died. I've found a few threads where the gas tube required tweaking but that's not my problem.
 

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12 hours ago, hondo said:

 I'm thinking about drilling out the offending port 1/32" larger which will lower the bottom of the "port" but only if I can get a spare barrel nut.
 

THIS.  Just cut it.  Measure twice, cut once.  Drill that fucker out and be done with it.

How much did this rail run you, anyway?  It would be real easy to get a rail that meets the specs that you need, but I can't wrap my head around why you're tied up on this one, and making it work. Just cut it.

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Due to my inability to buy extra barrel nuts for the two handguards I have I didn't want to risk messing one up modifying it so I ended up buying another free float handguard that uses a barrel nut that the gas tube goes over, not through the barrel nut. Unfortunately, the company,  AP Custom is out of business. A very nice, carbon fiber 9" octagonal tube. Barrel nut is torqued to 60ft.lbs., gas tube fits fine and extends to the middle of the bolt pin cutout, all is well.

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I installed a Miculek muzzle brake today so I could remove the upper from the vise it was occupying. Just lightly tilting the upper, muzzle down, allowed an assessment of the bolt carrier gas key to gas tube fit. Perfect, one finger into the ejection port sliding the bolt carrier back and forth, I was unable to feel any hesitation or snag as the gas key passed over the gas tube. I guess I had an underlying fear that I had an out of spec upper.

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