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Posted: 12/8/2014 12:19:40 AM EDT
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I just finished my first rifle build. It is the first AR that I've owned.
In case it helps, here are the parts Aero Precision M4E1 Enhanced Upper Receiver w/m4 feed ramps Aeroprecision BCG and Bolt AR15 Performance 223 Wylde Socom Fluted Barrel, mid-length gas system Aero precision low-pro gas block and tube DPMS Carbine Buffer DPMS Carbine Spring Magpul UBR Stock Magpul m3 30 rd Magazine loaded with 15 rounds Factory ammo - American Eagle 223 Rem 55gr All parts were 100% brand new, un-fired. I took it out and test fired it twice. On both shots the first round chambered successfully when racking back the charging handle and releasing it. After each shot, I attempted to take a second shot and got only the click of the hammer coming down on the firing pin. The second round failed to feed into the chamber after each of my manually charged shots. In both cases the first round ejected successfully. In both cases, the bolt returned to battery fully closed. I stopped at those two shots, simply because I didn't want to keep operating the weapon if I was having issues. I understand that this issue is most likely gas related? Things I didn't do: I did not record the angle of ejection of the fired rounds. I do not have the fired brass to inspect. I did not try shooting once with an empty magazine to see if the bolt locked back after the shot. I did not try another magazine. I did not try different ammunition. I did not try switching out the UBR buffer tube for the standard Milspec buffer tube. What I did do: Took my handguard off and re-checked the alignment of my gas block. The port in the gas block is exactly above the rear set screw, the port in the barrel is directly above the dimple for the set screw and I confirmed that the rear set screw is indeed sitting in the dimple in the barrel. This indicates that the gas block is lined up with the port. What should I try first? |
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You are asking the right questions. Prioritizing your approach will help.
First, did you clean the firearm thoroughly? Even though it is new, there is assembly lube and other foreign material that works its way into the system. A chamber brush and good cleaner lube like CLP is a good place to start. Did you put a little lube on all of the moving parts? Next, definitely load 1 round into a magazine. When the magazine is inserted pull down on the magazine to make sure it is securely engaged. That is a good function check for bolt catch, magazine release, and magazine function. Does the bolt lock back after that first shot? That will give you a better idea about gas. Is the magazine release screwed in enough so that the top of the threads is flush with the magazine release button? Then, do try another magazine. You should be able to show that magazine function and bolt lock are good on both or on neither, and it will confirm where the problem area is. If one mag works and the other doesn't, then you have an easy day. Also, check your magazine and make sure that spring and follower movement are smooth. Just because a magazine is new does not mean it works well to start. They often need break in and can benefit from a good visual check and cleaning and light lube. I am not seeing a gas problem to start. It sounds more like a magazine lock up or magazine problem. It might me a gas problem, but you are not down your check list yet. Please post how things are going. |
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Thanks for the reply.
Yes I disassembled and degreased the BCG then heated and lubed the parts with frog lube paste. I have not done any cleaning of the chamber yet, but I have a chamber brush and chamber mop, and will do so. I'll run through a few more tests and post back with my results. Thanks |
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Quoted:
Thanks for the reply. Yes I disassembled and degreased the BCG then heated and lubed the parts with frog lube paste. I have not done any cleaning of the chamber yet, but I have a chamber brush and chamber mop, and will do so. I'll run through a few more tests and post back with my results. Thanks Cool, run through that first list as much as you can before you get a chance to go to the range. In the meantime, if you want to confirm that your gas system is installed correctly, there are several things you can do to check it without being at the range. You already checked the gas block and it passed your test. Next, check that the gas tube is straight as it goes through the barrel nut. If it is tweaked even a little then gas block alignment on the gas tube adds possibility of dysfunction. First, visually check the gas tube alignment as it enters the receiver. Is it straight and centered? If not, then your barrel nut needs to be timed. You will probably pass this first check OK if the barrel was installed correctly. Then, check alignment of the bolt carrier and the gas tube. Do this by removing the bolt from the bolt carrier, then inserting the bolt carrier (without bolt) and seeing if it slides easily onto the gas tube with minimal if any resistance. That will tell you that the gas block on the bolt carrier and the gas tube are in the same line. Then check that the gas key is properly staked (probably yes) and that the bolt is proper in the bolt bore on the carrier. The traditional way is to hold the carrier and see if the bolt slides out too easily. If there is appropriate fit the bolt will not slide out since the rings will provide a little resistance. If this is all good then put your gas problem thoughts down the list of possibles. The spring and buffer should be good if they are carbine length and weight. Best. |
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Alright, I made it to the range this morning and have more data to report.
I fired 18 rounds of .223 55gr I used a brand new 10 rd. Magpul Pmag and a brand new 30rd Magpul Pmag I shot each shot with an empty magazine to test if the bolt would lock back on the empty mag. I then fired 20 rounds of 556 XM193 I used a brand new 10 rd. Magpul Pmag and a brand new 30rd Magpul Pmag I shot each shot with an empty magazine to test if the bolt would lock back on the empty mag. Each round I loaded, I loaded from a magazine with the bolt locked back, and then used the bolt release to load the weapon. I never had a single feeding issue on any of the 38 rounds fired. The bolt picked up the round from the magazine each time. On the .223, the bolt carrier partially locked back twice. In both cases, the bolt remained extended out of the bolt carrier group.and locked back behind the magazine in that orientation. Pictured: 
On the 556, the bolt carrier never locked back even partially. My .223 ejected brass occasionally was slightly dented at the opening of the neck, where it was bent flat. This was less noticeable with the 556. I have no idea what this means. The .223 case on the right shows the dent at the end of the neck. 
Thanks for the continued help and feedback. |
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OP,
Glad you got out to shoot! It does seem like a gas problem most likely. As in undergassed. It looks like your bolt is not cycling back far enough to lock properly. There are a couple of other possibilities but start by making sure your gas system is intact. Did you shoot any magazines with 2 or more loaded? If so, did they cycle correctly? I'm thinking no. Did you do the gas system check I outlined earlier? If not, do that now. FYI, the fact that your AR chambers and fires OK makes you OK to load and fire more rounds in the mag, you should be safe. Just keep it aimed at the backstop and keep your finger off the trigger until ready to fire. Ad just to be sure, it is the correct spring and buffer for a carbine, right? Best to you. |
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When you pull the charging handle all the way to the rear, the bolt should travel freely behind the back edge of the magazine about 1/8" or a little more. The wrong buffer, spring, or a shallow tube will not allow you to pull it back this far. It needs to go a little behind the magazine to allow time for the rounds to move up in the mag so it can strip one back off on the way forward.
The aero precision gas block is a set screw type, no? Double check that it is not canted on the barrel left or right or placed too far back to the shoulder on the barrel where the handguard cap normally goes. This will result in the gas hole in the barrel not lining up with the hole in the block. Use a caliper to make sure you have the gas block in the right place by measuring the distance from the back edge of the hole on the block to the back shoulder of the block, then the same on the barrel (back edge of the gas port to the shoulder on the barrel). Even with naked barrels and low profile gas blocks, the gas block will normally sit just a bit forward of the shoulder on the barrel. With any set screw type block, I would recommend a dimple in the barrel for at least one of the screws to keep it from moving. With most blocks, the hole drilled through for the gas tube lines up with one of the threaded holes for a set screw, making it very easy to do a dimple 180 degrees from the gas port on the barrel with the right jig. I bought one made by Black Rifle Engineering through either Brownells or Rainier and it makes this operation a snap. ETA: Post pictures of the hammer and underside of carrier in the firing pin area. An unshrouded firing pin mixed with a notched hammer will cause the carrier to hang up in the position you showed, but I don't think that is the root cause. |
I removed my gas system again to check for evidence of leakage around the barrel port. There is very little evidence of leakage there. The residue from the gasses do seem to indicate that the ports are properly aligned. There is some evidence of leakage around the gas tube where it enters the gas block. Though is doesn't seem to be significant. I don't think the gas block is the issue.
Quoted:
Check that the gas tube is straight as it goes through the barrel nut. If it is tweaked even a little then gas block alignment on the gas tube adds possibility of dysfunction. First, visually check the gas tube alignment as it enters the receiver. Is it straight and centered? Then, check alignment of the bolt carrier and the gas tube. Do this by removing the bolt from the bolt carrier, then inserting the bolt carrier (without bolt) and seeing if it slides easily onto the gas tube with minimal if any resistance. I had originally ruled out this option, because of the design of Aero's M4E1 upper receiver, which does not require the gas tube to pass through a timed gap in the barrel nut. However, I took a photo of the alignment of the gas tube inside of the upper receiver before removing the gas system. The tube was canted slightly to the right hand side, which caused friction with the gas key when the BCG slid into the upper receiver w/out the bolt. 
Re-installed the gas system with block rotated so there was no torque on the tube and it rested centered in the upper receiver. The friction between gas tube/key is eliminated. Perhaps this friction was the cause of my gassing issues; I don't know what else it could be. |
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Great, you have eliminated one possible source of problem.
Tobycreek had a great point, make sure that your bolt fully retracts with the use of the charging handle. Just ensures that you have full range of the bolt and not a problem with the receiver extension(buffer tube) or spring and buffer. True story: I bought a used assembled lower with a buffer and spring already installed. When I built the rifle it would mostly work but would sometimes show signs of short stroking. I couldn't figure it out for love or money. Finally, I took out the buffer and spring, and there was another buffer installed backwards so it acted like a spacer!! 2 buffers for the price of one, but a cause of intermittent malfunction. I removed the extra buffer, greased the springs, and reassembled everything and actually shot it all afternoon yesterday with nary a problem. The moral is, don't assume anything, and going systematically through your system will yield the answer. |
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Ok, so after re-adjusting the gas tube to eliminate any friction with the gas key, I took the rifle out again on Christmas. I had previously only lubricated my rifle with frog lube paste, but in the cold weather I decided to add some of the frog lube liquid as well and worked the action manually make sure it was nice and slippery.
I put 60 rounds through it. In that 60 rounds, the rifle fed the next round properly about 75% of the time. I probably had 15 failures to feed, about 7 of which were jams/partial feeds, and others just failed to pick up the next round from the magazine. The bolt never locked back on an empty magazine. This past weekend I took the rifle out again and put 80 rounds through it, each as a single shot. The rifle never locked back on the magazine. At this point, I feel that I should be past the break-in period, as I've not put nearly 200 rounds of various ammo through it, and worked the action while dry firing another 200 or so times. Assuming my gas system isn't the problem, what are the other causes of short-stroking on good quality commercial ammo? I've heard that a dirty chamber can cause brass to stick/resistance while bolt retracts for extraction. Anything I can do other than cleaning/lubing the chamber? Would this be causing the flattening of the casing neck on extraction like I've been seeing? It doesn't look like the rim on the base of the extracted brass is getting overly stressed by the extractor. There are markings but the rims of the brass are not bent backward or anything extreme. I also believe I read something about the hammer possibly causing resistance and slowing the bolt too much when it retracts into the receiver extension. How do I trouble shoot that? Any other causes that I should investigate? I may try clipping one coil off of my buffer spring and see if that helps. Thanks all! |
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If the picture of the bolt back is as far as it goes when you pull the charging handle back, then the buffer is wrong. The bolt should go back far enough to lock on the face of the bolt, not the ledge of the carrier. Take the buffer out and take some measurements.
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Quoted:
If the picture of the bolt back is as far as it goes when you pull the charging handle back, then the buffer is wrong. The bolt should go back far enough to lock on the face of the bolt, not the ledge of the carrier. Take the buffer out and take some measurements. The picture is just where the bolt locked back once when it caught on the magazine during firing. It retracts back farther when using the charging handle, and locks back properly behind an empty magazine then. The spring is 11.125 inches long, with 38 coils. The buffer is 3.25" long. Both of these are within the spec range for carbine parts, but the spring is on the long side. Perhaps buying a new carbine spring that's slightly shorter would work? Somebody recommended a Brownells brand chrome silicon spring. They measure 10.25" brand new. Would that help? |
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I hate to see someone struggle to get their rifle to work. You are doing all the right things.
About 20 years ago on the first AR-15 I put together, the BCG came assembled, looked like it was staked OK. But I had problems with functioning, not too different from the problems the OP is facing. The issue with my rifle was even though the gas key looked OK I found that one of the screws was stripped and I was leaking gas from the gas key / bolt carrier junction. In the thread, I saw only references to using P-MAGS, maybe try some good quality metal magazines. I once had a problem with a couple mags that would seat too high up and cause too much drag on the BCG. HK91 |
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Op, did you check the gas key for leaks? I have not checked my gas key for leaks but will do so. I have seen some folks mention a 45 degree test, meaning when held at 45 degrees, muzzle downward, the BCG should slide to the front of the upper. Can somebody elaborate on that? I assume the bolt should not be locking into the barrel extension just by force of gravity? Mine requires some force to get the bolt to slide back into the BCG, so the BCG stops there when tilted at 45 degrees. |
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I doubt your spring is the issue.
Do not clip the spring. It sounds like you are down to loose or broken gas key screw for undergassed troubleshooting. It is also possible, but unlikely that there is a blockage somewhere or the gas port is too small. The simple test is to use a bolt carrier from a working weapon. In the last few years I have only seen one case with a new weapon where it took more than the application of some extra CLP to fix what you are describing at the range. New weapons tend to be tight and too much drag will mimic undergassing or compound a situation where undergassing is actually a problem. In very cold conditions improper lube for the temperature or improperly applied lube is a possible cause of too much drag. |
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Thinking either tight head space, or a gas problem, but lets weed out a buffer or hammer wedging.
Pull all the way back on the charging handle, then look at the face of the bolt against the back edge of the ejection port window. The face of the bolt should be 1/8" to 1/4" in front of the back edge of the ejection port window, and during the last inch'ish of the charging handle pull, the tension should not have increased as you are pulling the handle back the last of the way. Gas leakage/blockage problem, CLP around the gas tube to gas block and around the front and back of the gas block to barrel. Now using a tube and air compressor, pressurize the gas tube from inside the receiver, and look for air leaks between the tube to block, and the block to barrel. Next, CLP around the base of the carrier key to the carrier, hold the bolt in, then pressure the front of the carrier key to check for leaks between the base of the key to carrier. If the block is leaking around the gas tube, or the barrel, here is your problem. If the key to carrier, then here is your problem. As for head space, need a head space gauge to check that, since too tight of head space will create over pressure problems and the higher the ammo working pressure, the more the rifle is going to over function. Lastly, with the rifle cycling lower pressure 223 semi normal, and not higher pressure 5.56, does sound like a over pressure problem. If the rig comes up leak free, see if you can borrow a H-3 buffer to weed out the barrel gas port being drilled too large and causing over function problems. |
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Thinking either tight head space, or a gas problem, but lets weed out a buffer or hammer wedging. . Can you elaborate on the hammer wedging? I tried to find more information but search engines keep bringing up the Accu-Wedge product. I understand that if the hammer is causing too much resistance when it resets, it could cause the bolt carrier group to slow down. How can I test for that, and how is that fixed? Would polishing the face of the hammer make any difference? Thanks |
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Dimensions of the rifle are off, and as the carrier comes back, the hammer is wedged between the tail of the trigger, and the bottom of the carrier.
To see signs of this, look for a white line/denting of the hammer tail to the back of the trigger tale. Also look for sighs of the top of the disco hook making hard contact to the back of the hammer as well. Lastly, look for signs that the trigger tail is coming down to make contact to the trigger hook surface just before the tail. Bluntly, the disco tail acts like a buffer blow surface for the trigger tail when the hammer comes back on over cock to not slam into the trigger tail hard. If the hammer tail is too long/dimensional problem, then the hammer end up wedging between the bottom of the carrier and hammer to cause a wedging problem that cause short stroking. Worse yet, you have a dimensional problem, where the tail of the hammer is crashing into the back hook surface, instead of the disco tail. Just post a photo of your trigger tail area with the hammer un-cocked, and I can tell you if you have hammer wedging going on. |
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Quoted:
In the last few years I have only seen one case with a new weapon where it took more than the application of some extra CLP to fix what you are describing at the range. New weapons tend to be tight and too much drag will mimic undergassing or compound a situation where undergassing is actually a problem. In very cold conditions improper lube for the temperature or improperly applied lube is a possible cause of too much drag. So after another trip to the range this weekend, this comment is starting to sound more on point. I cleaned my rifle and lubed it heavily prior to heading the the range. I got through 20 rounds on 55 grain .223 American Eagle without a single failure to lock back on an empty magazine. I then fired through 40 more rounds of .223 AE 62 grain, with intermittent failure to lock back on the mag (perhaps a 20% failure rate). So that was a huge boost to my confidence. I guess I'll continue to shoot this thing and lube it up and see if things continue to improve as it breaks in. Thanks to all for the help so far, much appreciated! |
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