Newly built AR is short stroking/wont cycle
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Newly built AR is short stroking/wont cycle


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Old 04-07-2014, 11:51 PM   #1
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Default Newly built AR is short stroking/wont cycle

Hey guys, having malfunctions with my new rifle and I'm BIG TIME bummed out about it. First time I ever shot a semi auto rifle that wouldn't feed (and turned into a single action rifle as a result)

Just built a carbine length Matrix Mil Spec upper, Yankee Hill gas block & tube, DPMS bolt assembly, Colt Socom 16" 1-8 twist and a 10" quad rail. Married it to a Aero precision complete lower. The fit is PERFECT.

Anyways, just took it to the range over the weekend and went to fire a 3 shot group to zero my scope and the 3rd shot went CLICK and no BANG. I waited 20sec, dropped the mag and charged the bolt back and sure enough....it didnt grab a new round and continued this way for the following 6-7 shots. I packed it up and went home to inspect it. And to answer the inevitable questions, I used 2 different 20-rd PMags (that functioned flawlessly on another rifle) and PMC 55gr .223 ammo in both mags. I also had a stovepipe and a light primer strike. Like I said, after the first 2 shots it went all out of wack.

I am going to create a checklist and run down each line item starting with the gas system until I isolate teh variable. Any suggestions on how to go about this in the most SANE & logical way possible?? lol

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Old 04-08-2014, 12:13 AM   #2
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What weight recoil buffer did you use? May need to step up to an H buffer which is a bit heavier than a standard carbine buffer.
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Old 04-08-2014, 12:15 AM   #3
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Stainless,

Obviously it is hard to diagnose an issue not seeing everything. But I would suggest a few things.

1. Find a friend that has an AR and put your upper on his lower and see what happens. This is an attempt to isolate any dimension issues with parts form various manufacturers.
If your upper works good on his lower we can address that next! Like wise if his upper works well on your lower that also leads us to a solution.

2. Be sure if the Bolt Carrier Group has a "'window cut" in the hammer and you can see most of the rear of the Firing Pin in the open including the Firing Pin Flangeramp on the bottom of the carrier be sure you do not have a large flange firing pin in it. Are there any strike marks on the bottom of the carrier caused by contact with the hammer and the carrier. Also check the Hammer for wear or strikes that should not be there being a new rifle. If it has the window cut it should have a small flange firing pin in it. Small Firing Pin Flange diameter .325 while the large flange is .375. I experienced this with a guys rifle last week and that was the problem he had the wrong firing pin in the carrier.
3. If the same problem exists and you are sure the mags are good (also try another maybe your friend has) Then we may be looking a t a gas problem. The gas block possibly blocking part of the gas port. As far as the light strike on the Primer that is normal if a round has been chambered. It is an enertia firing pin and will put a slight dimple in the primer anytime the bolt has been dropped on the round in the chamber. Nothing wrong there! Let us know and we can go from here. Rather than just guessing we need to start eliminating areas that might be the cause. Let us know what you find out.

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Old 04-08-2014, 12:34 AM   #4
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MUCHO thanks guys. I have a few scenarios to play out to isolate the issue. And yeah the BCG has a nice wide cut and worked FLAWLESSLY in my DPMS Oracle. I swapped that bolt into my new setup and have to inspect it very closely to look for issues. Might be as simple as worn gas rings.

But yeah to answer another question above Gator, its the stock carbine buffer that came with the Aero lower. Somewhere in the 2.5 ounces neighborhood I believe. I just ordered a Spike's T2 buffer which is in the middle weight wise. I think it jumps the weight up to 4oz. Either way I know it reduces recoil and vibration so its nice to have anyways.

Thanks fellas. Ill keep you updated.....
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Old 04-08-2014, 02:21 AM   #5
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Just checked the entire gas line and its perfect from the block to where it mates with the bolt gas key. I inspected the block and it was rock solid.....then I put it back together, covered up the chamber with a finger....left a small piece of fabric in front of the gas tube in the upper....and blew on the muzzle brake and it was a nice even wind tunnel that blew the fabric around in the upper. So I know the gas system works.

I then moved to the bolt. Inspected the gas rings and they were still nice and tight. I grabbed the bolt and did the shake method and the bolt flew out with some force....then I placed the bolt upside down to see if the carrier weight would force the bolt face to shut, and it stayed extended. So I know the rings are good. I then sprayed a bunch of Ballistol around the gas key base, and blew on the gas port with a straw to check for air bubbles escaping from the bottom. I saw a coupe very tiny bubbles emerge from the base of the gas key area. I know that SOME gas is escaping there but not enough to concern me.

I did clean the chamber and feed lips very well and lubed everything up good... which i did NOT do the first time I fired it.

So now this leaves me with 2 other strong possibilities: the buffer is too light or my rifle has a strict diet and needs 5.56 ammo with more pressure.

Any thoughts on my findings tonight? Or my gameplan moving ahead while I test out various scenarios?
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Old 04-08-2014, 03:48 PM   #6
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You might want to check the barrel nut alignment. If it is off you could be leaking gas at the bolt carrier key.
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Old 04-08-2014, 09:54 PM   #7
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dpms bcg not the best out there and some can be problematic in improperly torqued and non staked gas key bolts. gas can bleed around the key base. ive had to do that to a few dpms carriers. if you do retorque and stake the bolts be sure to clean the base of the key and and the carrier of all carbon before putting the key back on.

between the key and gas block and maybe too heavy buffer it could be enough.

another cause is if you stuffed a rifle length recoil spring in in a carbine buffer tube...
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Old 04-09-2014, 01:21 AM   #8
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Quote:
Originally Posted by JonM View Post
dpms bcg not the best out there and some can be problematic in improperly torqued and non staked gas key bolts. gas can bleed around the key base. ive had to do that to a few dpms carriers. if you do retorque and stake the bolts be sure to clean the base of the key and and the carrier of all carbon before putting the key back on.

between the key and gas block and maybe too heavy buffer it could be enough.

another cause is if you stuffed a rifle length recoil spring in in a carbine buffer tube...
If you do I suggest you replace the key AND the screws.
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Old 04-13-2014, 04:38 PM   #9
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What about lubrication ? Is the bcg well lubed ? Is there any lubrication in the buffer tube ? Every little bit of drag can make this happen . I habe seen numerous ARs with issues just from not having any lubrication . On came to me that still had the factory sticky goo on the bcg and it wouldnt even drop all the way in . I asked him if he has ever cleaned it and lubed it . His reply was its only had less than 10 shots it shouldnt need it
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Old 04-13-2014, 06:51 PM   #10
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Here's something no one has mentioned that I have seen in rifles customers have brought me to fix. If your buffer tuber is screwed on one thread too many it will short stroke. Make sure your buffer tube is barely holding your buffer detent into your lower receiver. The edge of your buffer tube should be on the edge of the detent. If it's resting up against the nipple of the detent and even pushing the detent forward, you need to back off your buffer tube a thread.
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