bgary Posted March 5, 2012 Share Posted March 5, 2012 I was at the range yesterday with an assortment of carbines. Had fussed with sights on them over the off-season and wanted to check/restore zeros. One of them is an upper I bought used, sitting on a lower I built. So... carbine #1 runs great, carbine #2 runs great... everything going fine right up until I get to the "new" one. Got 4 bolt-over malfs in the first 10 rounds fired. Got about 30% bolt-over failures in the remaining 25 rounds I ran through it before putting it away. It seems to me there must be something wrong with the bolt, because... -- I only had one kind of ammo with me, and it worked in the other rifles, so... probably not ammo -- I only used one mag through all the testing, it worked in the other rifles, so... probably not mag-related. -- I tried swapping out lowers (different buffers, springs), same malf with all of them, so...probably not spring/buffer related. So if it is the bolt, I'm wondering what I should look for. This is a bolt that came with the upper, sitting in a new Daniel Defense carrier (seller kept the original carrier). I'm wondering if the bolt has an issue, or if it doesn't like the carrier it is in, or... something else? Next time I go to the range, I'm thinking of swapping in a known-good JP bolt-and-carrier from another rifle, on a known-good lower, and seeing if the problem goes away. If it does, great. Is that a bad idea? Anyone have better ideas? if that doesn't solve the problem, not sure what to try next. Thx... Link to comment Share on other sites More sharing options...
stein Posted March 5, 2012 Share Posted March 5, 2012 (edited) Did you test to see if the "troubled" carbine locked back on the last round? My first thought is that it's not cycling far enough rearward(under gassed?) to strip the next round. EDIT: that "other thread" has some good info-probably better than mine Edited March 6, 2012 by stein Link to comment Share on other sites More sharing options...
Smokecloud Posted March 9, 2012 Share Posted March 9, 2012 Short stroking is generally always a gas issue. Since you have done quite of bit of swapping parts, I would give your gas system a thorough once over. Make sure the roll pin is in place on the front sight base that holds the gas tube. see if there is evidence of leaking around the pin area. Check the back of the gas tube in the upper receiver, does it look deformed or different than your other two guns tubes? Next check the gas "key" on top of the bolt carrier, is it loose? If its not properly staked, can you tighten the allen head fasteners? If its loose, that would definitely be the problem. is the gas tube or Key blocked by anything? make sure there is no q-tip head or anything like that blocking the path. Aside of that, 90% of all AR feed/cycle problems are mag and ammo related. I have a couple that will run even most junk mags, but one of my AR's is very picky about the mags it likes. Make sure your bolt and carrier are well oiled, especially when new. Run standard carbine action springs and the correct weight buffer if you built it. A lot of people buy buttstock kits that include springs and buffers, most of those buffers and springs are either non-standard stuff or cheap and dont weigh what they are supposed to. Not sure what your skill level is with AR's, but if all of that passes, I would be looking at the port size and hole line-up on the front sight base, the FSB is usually considerably larger than the port hole in the barrel, I would have to look up the port hole size to give you an accurate spec, but anything less than minimim specs, would definitely cause probs. The last AR I put together for a friend that had bought a "bargain" parts assembly, short stroked most every round, I ended up locating the problem after several tests, it ended up having an out of spec FSB, swapped it out and it ran fine. Link to comment Share on other sites More sharing options...
bgary Posted March 9, 2012 Author Share Posted March 9, 2012 Solved... or at least, neutralized. I had tried this upper on a variety of known-good lowers (I left the upper alone as a "control", because I wanted to narrow down whether it was a spring/buffer issue or a gas/bolt issue). Anyway, it did the same thing no matter what lower it was on, so I figured it wasn't springs, buffers, mags or ammo. So... I swapped bolt carriers. I took the bolt out and put it into a JP low-mass carrier, fired it up and... it runs like a top. 150 rounds through it with no hiccups, and no changes other than taking the bolt out of one carrier and putting it in another. So, the Daniel Defense carrier is on the bench for now. At some point when I have "spare time" I'll see if I can figure out why it barfed (too heavy for the gas coming through the tube? different geometry? Sunno) But the main thing is, the rifle is running now. Thanks for all the great ideas! Link to comment Share on other sites More sharing options...
Kevin G. Posted March 9, 2012 Share Posted March 9, 2012 Solved... or at least, neutralized. I had tried this upper on a variety of known-good lowers (I left the upper alone as a "control", because I wanted to narrow down whether it was a spring/buffer issue or a gas/bolt issue). Anyway, it did the same thing no matter what lower it was on, so I figured it wasn't springs, buffers, mags or ammo. So... I swapped bolt carriers. I took the bolt out and put it into a JP low-mass carrier, fired it up and... it runs like a top. 150 rounds through it with no hiccups, and no changes other than taking the bolt out of one carrier and putting it in another. So, the Daniel Defense carrier is on the bench for now. At some point when I have "spare time" I'll see if I can figure out why it barfed (too heavy for the gas coming through the tube? different geometry? Sunno) But the main thing is, the rifle is running now. Thanks for all the great ideas! Putting the LMOS bolt carrier in a malfunctioning under-gassed upper is one way to "fix" it. Link to comment Share on other sites More sharing options...
assaulter Posted March 10, 2012 Share Posted March 10, 2012 Check the gas rings in the problem bolt. Link to comment Share on other sites More sharing options...
Smokecloud Posted March 10, 2012 Share Posted March 10, 2012 I thought about the gas rings, but forgot to type them into my last post, thanks for catching that. If the bolt carrier is truly the culprit, then its probably the gas key that is giving you fits. you might try swapping bolts into your old carrier and see if you can eliminate it further, you still dont know if its a bolt/gas ring, or carrier/key issue. Link to comment Share on other sites More sharing options...
dmc_md Posted March 11, 2012 Share Posted March 11, 2012 Put the "bad" bolt carrier into one of the other guns and see how it runs. If it doesn't work in a known good gun then it's a problem with the bolt carrier. If it works just fine, it's a problem with the upper's gas system. Darren Link to comment Share on other sites More sharing options...
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