robtmc

I did find an old post from a guy had the same oversized thread problem. Tolerance stacking. Funny because the Midwest barrel nut fits fine with just a touch of drag in a few spots. Tried a JP HG years ago with the tight thread problem and sent it back. Had forgotten that.

Bought the upper in 2007, not 70's. Not cross threaded, can get it on with some effort, not cross threaded, the Midwest nut goes on by hand still.

Wanted to replace my old Midwest handguard that had to be loctite bonded to stop slippping. Mid-70's DPMS LR308 receiver. Got an Aero S-ONE FF handguard with barrel nut claimed to be "ar308" compatible. DPMS is mentioned. No way, started a thread or so then needed a wrench to got any further. NOT cross threaded. The Midwest nut goes on by hand. Aero claimed lack of commonality with the 308 and claimed nothing they could do. I have trouble believing the upper receiver threads are that mismatched. I know Armalite is different but should not have even started if that was the case? Aero's description for just the barrel nut does state only for their threaded upper receivers.

You are aware the AP4 barrel is a hefty little chunk at 2.5 lbs, aren't you? Might consider having it turned down if going for light weight.

Is that the SS or lightweight CL barrel? I just rebuilt my rifle with the CL barrel and really like the way the balance and feel have changed from the 16" DPMS AP4 barrel. Now to get my thumb out and see how accurate it is. Just waiting for RRA to ship their 1 piece scope mounts.

Troy TRX barrel nut, and RRA one piece scope mount.

Typing too fast. The old barrel was 16". .063, opened to .067. That one worked great opened up. The 18.5" is unfired, has a .093 port, hopefully plenty large. Will engage brain next time.............

Can only say what the 18.5" Fulton Criterion barrel is. .093" or a #42 drill. Can't say how well it cycles in rifle length, have not gotten the gun to the range again. That said, drilling a partly blocked .093 carbine hole to .097 gave perfect function, to the point that the JP adjustable block could be turned way down to limit brass throwing. New JP trigger waiting to be installed, will not make the crappy trigger mistake again while trying for group size.

Guy from Troy got back to me, no replacement nuts to send out: "Currently we do not have any more barrel nuts because we had an issue like yours" So they are aware of the problem, sounds like a batch of poorly machined parts. Stand up service, not arguing the point like so many companies.

Yeah, the specs I found show a lot of room for the min and max on threads. Not at all clear, and in this case, seems there is no "mil-spec" for anyone to follow. Hell, maybe that is the difference between Armalite and DPMS, thread %? Thread diameter looks like 1 3/8", maybe -16. Crazy if so, why do that?

Glad Troy is sending out a new nut. Best I can tell, the problem is the threads were too shallow. This is not the best macro shot, but the flattened tops of the threads are very clear under magnification. Explains the binding.

The guy at Troy suspects the wrong nut too. The difference must be miniscule. He said the threading on a thread or two was a good indicator. Said the Armalite nuts used to have a small "A" marked on them. No marks on this one, so I hope this is the problem. New nut on the way. As far as I can tell with calipers, this thing should thread. Thread pitch, major and minor diameter all are extremely close. Suspect it is one of those % engagement things, where the drilled hole is slightly undersize to increase the thread engagement? Funny, as the two DPMS nuts I have are slightly different, one goes on by hand, the other needs a wrench to go fully on. Large diameter threads must be a problem to form. Would love to see the specs on the threads for both, and understand why in hell DPMS would choose some slightly different spec. No meaningful competitive edge to that.

Been out in the garage, stripping down my LR-308 of it's AP4 barrel, FF quad rail. Task is to install a new Criterion 18.5" barrel and Troy TRX FF tube. Bit of history: originally had a DPMS carbine FF tube on with 16" barrel, nut was a fine fit on the threads by hand. Easy to torque smoothly. Replaced it with a DPMS quad rail, and immediately notice the nut is much tighter on the receiver threads. Got it together, but was concerned the tight threads threw off the barrel torque setting, fighting the friction. Nut cam back off fine with the DPMS wrench, but not as tight as I thought it would be. Kind of supports my feeling the nut might not have been properly torqued. Now I go to put on the Troy barrel nut, and it is ridiculously tight, more so than the DPMS quad, so much that the Troy wrench (shaped like a "C") spread open enough to skip long before the nut was bottomed out. Squeezed the wrench back enough with a vise to remove the nut, but this is not right. I have seen pics of Troy wrenches that are closed, not open, but that is what Viking Tactical sent me. I have a PRI wrench that cannot spread being closed, but the center hole is too small to go over the barrel. Thoughts on this? About ready to put the AP4 barrel back on so at least I have a functioning rifle. Did wire brucsh the receiver threads, no burrs or anything, and the old DPMS nut still spins on easily. Addendum: Smeared some metal polish on the threads and working it back and forth. Still barely 1/8" of engagement before it get very stiff. You can feel the friction heating up the nut. This is a crock considering the cost of these parts. Troy of course is closed long before I found this, so no call to them yet. The fact that two DPMS nuts go on either freely or with a bit of friction tells me the receiver is withing spec, as I suspect that will be the first Troy response.

I started shooting with the GB out 4T. From peering in at it, this is WFO. Funny thing is, there is precious little perceived feel to the rifle from WFO to 1T. Only the cycling changed. Suppose it is the dinky little 16" barrel and the .308. Guy next to me was laughing at the dust puffing up from the shock wave hitting the ground, and I was sitting, not prone. It is a loud little sucker. Next up is switching over to the Criterion 18.5" barrel. Any change in character will be interesting.........

After clearing out any burrs in the port, I ran an .067" drill through to make sure the hole was cleaned up. Bore brush cleared out any burrs I might have seen hanging into the bore. Went out to shoot it today, and it works fine. Ended up with the JP adjustable GB at 1.5 turns out. It actually would lock back fine at 1 turn, so went down to 0.5 and had a failure to lock. Went back to 1 T and had a failure to eject. Back to 1.5 and it ran like a top. Brass lands in a 2' circle, Since I cannot leave well enough alone, I did the infamous "15 minute trigger job" while things were torn down. NATO ammo does not like the lightened hammer strike. Commercial .308 worked fine. Live and learn...........

I vote for Dremel and wire brush. Carefully to not nick up the rest of the receiver. Lots of duct tape to protect stuff. I shudder to think what a tap/die in the thread size of the receiver would cost............

I got to reading about this, being someone with a carbine length system and feeding issues. My take was rule of thumb is around 6-7" of barrel past the port for reliable dwell and operation. Gas port size is also a variable, which may have been my problem with a 16" barrel and small .063" port. What I read is that other dwell times can be tuned for with gas restriction or opening if it comes to that, with a limitation on orther loads that cycle well. Very short barrels were the prime culprit. Can be tuned for one load, but others are in the twilight zone for reliability. Read about someone wanting to cut down a 24" with rifle length gas and asking about problems. YMMV, that was the take home for me about barrels and gas port/size location. Sounded like more art than science.

Just to be contrary, and out of curiosity, why do you want a mid-length gas system with a longer barrel? From what I have read, there is an optimum length of barrel beyond the gas port for optimum dwell time, and a 20"+ barrel may be out of bounds here.

Nice idea, went to order some, but they are rigidly attached to UPS ground shipping, so a 8$ pack of washers that could go in a 1st class envelope requires $20 Ground shipping to HI? I hate these outfits that will not give you alternatives for shipping method and end up charging more for shipping than the damn parts cost......... /Rant off

For more reference, a few weights. The ACE skeleton is said to be a lightweight alternative to the A2 stock, and it may be. But, it is heavier, with rifle tube, spring, and buffer, than an AP4 carbine stock, with RE, spring and buffer: ACE/rifle tube/spring/buffer 1lb 5.9oz AP4/spring/buffer 1lb 3.1oz My Upper with DPMS quad rail and all guts in place, 16" AP4 barrel, DD rail mount sling attachment and rail covers still on. Gas block is a JP A2 sight adjustable: Upper with 16" BBL/quad rail 6lb 2.56oz The JP A2 GB is spec'd at about 4oz, a railed GB I have on hand is 2.6oz. For me at least, any significant weight change will be going to the Troy TRX handguard. It weighs 13oz with nut. Considering I am going to a rifle length barrel and HG, looks like it will still be lighter than the carbine setup. A Miculek compensator I have is 4.1ox, an A2 flash hider is 1.0oz, both with lock nut or washer. Playing around in a spreadsheet with all this, but for me, looks like it comes down to how much weight comes off with the DPMS quad rail. No huge areas of excess to be removed as Glockslap has already shown. Great fun regardless if you are mechanically inclined...........

Aw hell, that is only ounces more than the Criterion 18.5" CL lightweight. Was hoping the AP4 would be around 3lbs, and make it worthwhile to tear things down and swap it. Maybe just shoot the 16" out and save the longer barrel or build another upper.

Ok, so what does the AP4 barrel weigh? Will post a few weights as I re-do my rifle. Really curious how much the DPMS quad rail is, looks like an awful lot of excess aluminum in the thing, compared to the Troy.

The only thing concentric to anything else on a barrel is bore to outer barrel. Cutting it shorter ha nothing to do with that. Getting the contour cut down to save weight would be the issue, though I suspect good machinists know how to do that right. After all, the barrel blank is chucked up in a lathe to bore it in the beginning. No personal experience with them, but heard good things about an outfit called ADCO for barrel re-profiling.

Saw nothing for us island AR shooters. Fact is, most are on Oahu, and are 5.56 guys. There are .308 guns around, just not so many. May be a shock, but AR rifles are far less restricted here than in California. Biggest issue is a 10 round mag limit that no one pays attention to. No removable flash hider, collapsible stock, or whatever crap. I am on the Big Island, with no formal ranges, but a few guys I know that shoot small caliber AR. Good pig hunting here, should I get my lazy arse up for that.......

Tacking on for replies. I have the same issue with my DPMS AP4 build. Using a JP adjustable GB and went through the 1.5 turns out and more. With it in too far, failure to eject. Further out ejected great, but no feed, and no lock back on empty. Two DPMS mags and a Magpul all the same. Locked back and fed manually just fine. Took the gas block off yesterday, carbon pattern perfectly centered on the gas port, so that is not it. Decided to measure the port hole size, .063". What was interesting was the drill bit did not just slide into the barrel, but hit an obstruction. Maybe a burr from the drilling? Pushed it through the obstruction and checked the bore. Looks fine. What is odd is the port size vs. a Fulton 18.5" barrel with rifle length gas system. That has a .093" hole. I find the hole size thing rather confusing. Lubed the heck out of things and will try again, after making sure the gas tube is clear.