hondo Posted February 17, 2020 Report Share Posted February 17, 2020 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. Quote Link to comment Share on other sites More sharing options...
blue109 Posted February 17, 2020 Report Share Posted February 17, 2020 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 Quote Link to comment Share on other sites More sharing options...
98Z5V Posted February 18, 2020 Report Share Posted February 18, 2020 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. Quote Link to comment Share on other sites More sharing options...
hondo Posted February 18, 2020 Author Report Share Posted February 18, 2020 When I typed, "The barrel nut looks like a DPMS barrel nut with 22 holes." I should have typed a DPMS pattern barrel nut with 22 holes. It looks like this only with 22 holes. 748354.wdp.jxr Quote Link to comment Share on other sites More sharing options...
hondo Posted February 18, 2020 Author Report Share Posted February 18, 2020 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. Quote Link to comment Share on other sites More sharing options...
blue109 Posted February 18, 2020 Report Share Posted February 18, 2020 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. Quote Link to comment Share on other sites More sharing options...
COBrien Posted February 18, 2020 Report Share Posted February 18, 2020 @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? Quote Link to comment Share on other sites More sharing options...
hondo Posted February 18, 2020 Author Report Share Posted February 18, 2020 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. Quote Link to comment Share on other sites More sharing options...
98Z5V Posted February 19, 2020 Report Share Posted February 19, 2020 Pics, man. Pics. Don't know what that was that you attached previously, but it wasn't a picture format that can be viewed on a computer. Quote Link to comment Share on other sites More sharing options...
hondo Posted February 19, 2020 Author Report Share Posted February 19, 2020 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. Quote Link to comment Share on other sites More sharing options...
98Z5V Posted February 20, 2020 Report Share Posted February 20, 2020 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: 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? Quote Link to comment Share on other sites More sharing options...
hondo Posted February 20, 2020 Author Report Share Posted February 20, 2020 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. Quote Link to comment Share on other sites More sharing options...
98Z5V Posted February 21, 2020 Report Share Posted February 21, 2020 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. Quote Link to comment Share on other sites More sharing options...
hondo Posted March 1, 2020 Author Report Share Posted March 1, 2020 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. Quote Link to comment Share on other sites More sharing options...
hondo Posted March 16, 2020 Author Report Share Posted March 16, 2020 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. Quote Link to comment Share on other sites More sharing options...
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