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Old June 24, 2013, 09:32 PM   #1
Join Date: May 30, 2012
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AR Issues

Trying to help my cousin troubleshoot his AR. We went shooting yesterday and the first shot he fired was followed by a slamfire. There would of been a third but the primer didn't ignite but it was dented. A few rounds later he had a case stuck and the bolt fed another round behind it pushing the bullet 1/2" down into the case. The case's rim didn't have any damage from the failed extraction if that matters. Some background. It's a built AR meaning it wasn't bought whole from a GS and the Barrel is chambered for 5.56mm. He was shooting Winchester 5.56.
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Old June 25, 2013, 02:21 AM   #2
Navy joe
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Sounds like it is short cycling to me, why is the big question. Crap ammo, wrong buffer/tube, incorrect assembly in bolt carrier, bolt or buffer. Best bet is to have someone who knows ARs, as in a gunsmith or armorer go through it before any further firing.
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Old June 25, 2013, 04:43 AM   #3
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slam fire is probably something wrong with the trigger group, disco not catching hammer, may not be assembled right, or something is out of spec.
the stuck case may try cleaning chamber first. could be an extractor issue may need an upgrade kit.
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Old June 25, 2013, 10:02 AM   #4
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Did it pass the function checks?

If his response is "What function checks?" then he missed a major part in making sure the thing is safe to even take to the range.

AR fire control group function check procedure*:
1) Cock the hammer, turn the safety on. Pull the trigger. The hammer should NOT fall.
2) Turn the safety off. Pull the trigger. The hammer SHOULD fall.
3) With the trigger still depressed, cock the hammer. The hammer should be held by the disconnector.
4) Release the trigger. The disconnector should release the hammer to be caught by the trigger.

*I prefer to do this with the upper off the lower so I can see what is going on, but DO NOT let the hammer just fly forward and hit the receiver- that can damage things.

I had a fire control group from a reputable manufacturer (Daniel Defense) fail at #4- the trigger would not catch the hammer. That would mean you'd have it firing twice with each trigger pull/release. Not a safe condition. Turned out that it was two parts on the ends of the tolerance specs... any other hammer or trigger and it worked fine. That particular hammer/trigger together... problem. Daniel Defense is a stand up company and replaced the hammer, trigger, and disconnector for me for free. The replacement unit did work fine.

Now, it's not a guarantee that this is your problem, but if the fire control group passes these checks then the issue isn't in your lower.

You can then check out the bolt carrier group. I wonder if there isn't some issue in the firing pin (or the firing pin channel) keeping the pin from floating to the rear when the action locks closed (there's no spring keeping the pin off the primer, dented primers in an AR from just cycling the thing without firing are not unusual.

If you had a situation like I had, then the slam fire would have been from letting off the trigger and the dented primer on the 3rd round would be normal. If you have an issue with the firing pin/bolt carrier then for whatever reason the firing pin floated free and kept it from burning through the whole mag on full auto.
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Old June 25, 2013, 10:54 AM   #5
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Re: AR Issues

I'm guessing that he bump fired the second round. The third round wasn't actually hit with an intentional primer strike, it was touched by the floating firing pin. Any round that you chamber in an AR will have a small indentation in the primer because the firing pin is not held tightly in a rearward position, it is allowed to "float" with a little wiggle room.

As to the case stuck it could mean a couple things. He could have an extractor problem, or the rifle could be over gassed and the BCG is bring pushed too quickly to hold onto the case. The fact that the rim of the case looked undamaged however makes me think it is probably the former.

Can you give us a quick parts list? Barrel, gas system, bolt, bolt carrier, buffer?
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Old June 25, 2013, 12:52 PM   #6
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To the OP, Technosavant is 100% correct in his function check warning, but since I do not know the level of competence/knowledge you have in the AR, let me change some of the words.

1) Pull the charging handle fully to the rear and release, turn the safety on. Pull the trigger. The hammer should NOT fall.
2) Turn the safety off. Pull the trigger. The hammer SHOULD fall.
3) With the trigger still depressed, pull the charging handle fully to the rear and release. The hammer should be held by the disconnector.
4) Release the trigger. The disconnector should release the hammer to be caught by the trigger. There should be an audible and tactile "thunk" when this happens if you are using a GI trigger group. Aftermarket ones, your YMMV WRT tactile/audible "thunk"...

As for the double feed... it sounds like the round was not extracted by the BCG by the time it went to the rear, then on the return it stripped the next round and jammed it as you desribed. Did this happen during the middle of a magazine, or after loading? I ask because I've seen some folks ride the charging handle forward, where it did not engage the lugs, hence the cartridge rim was never engaged by the extractor. When user pulled the trigger, nothing happened. They then proceeded use a "slap, rack, squeeze" technique, and never looked at the chamber. By slapping the magazine to ensure it was seated, then racking the charging handle again, they created their own double feed. Real pain to clear, as you probably learned.

If the double feed happened in the middle of a magazine, then it could be the extractor, it could be short stroking, or if both rounds were not fired, it could be a faulty magazine.

And dimpled primers are normal in the AR. As mentioned, since the firing pin is free floating in the BCG, as you pull the charging handle and release, or hit the bolt release, the momentum of the BCG slamming forward is enough to leave a light dimple on the primer.
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Old June 25, 2013, 01:28 PM   #7
Join Date: May 30, 2012
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AR Issues

Thanks guys. I'll forward it to him. I'm familiar with performing a function check. Was one of the first things I was taught in the Marine Corps during weapons training.

My cousin lives an hour away and I don't have access to his AR. With malfs like this in the Marines the weapon goes straight to an Armorer to perform that echelon of maintenance. I can tap, rack bang with the best of em. But the furthest we were allowed to disassemble was taking the buffer and buffer spring out as well as removing the bolt and firing pin from the bcg. The bump fire happened on the first firing. He fired another 20-30 rounds without issue. Until the stuck case that is.

Last edited by nmbrinkman; June 25, 2013 at 01:33 PM.
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Old June 25, 2013, 02:17 PM   #8
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If your cousin is using a carbine upper...he needs to use a carbine buffer spring. Same goes if he's using a hydraulic buffer. Make sure the gas key on top of the bolt carrier is tight, and the gas key bolts should be preferably staked; to keep them from coming loose.
That rifle hanging on the wall of the working class flat or labourer's cottage is the symbol of democracy. It is our job to see that it stays there."

--- George Orwell
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Old June 25, 2013, 03:18 PM   #9
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the function check doesn't always show a problem if you still get the occasional slam fire its probably still in the fire control group regardless if it passed the function test.
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Old June 27, 2013, 02:08 PM   #10
Bartholomew Roberts
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Making an AR15 slam fire is exceedingly difficult due to the design. Short of something in the firing pin channel wedging it forward or some really high seated primers, it is one of the few AR malfunctions I've yet to witness first hand (knock on wood). My guess would be doubling due to an issue in the lower.

But if you are getting stuck cases with brass cased .223 pressure ammo, you've got another issue on top of any problems with the lower. Who built the rifle and was it a factory build or frankengun?
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