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Old June 8, 2013, 05:51 PM   #1
k511
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AR pistol short stroking

My good friend just purchased an AR pistol and its having ejection problems...I don't know much about the platform so I figured I would turn here for recommendations... its a spikes punisher lower and a Yankee hill upper 7 inch barrel...thanks in advance
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Old June 8, 2013, 05:58 PM   #2
Nathan
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What ammo? Is the upper completely YHM?
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Old June 8, 2013, 06:38 PM   #3
k511
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It is all YMH except a bcm extractor....we were using cheap PMC ammo, I'll have to get back to you on what grain was
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Old June 8, 2013, 09:30 PM   #4
insomni
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The whole barrel is 7"?
I'm inclined to say it's a gas system problem (especially if your ejector is a new BCM ejector). if you dont have enough gas pressure in the tube, the bolt's gonna short stroke. I know 10" barrels are about as short as pros like to go (I mean actual shooters, not sponsored plinkers) and even then the stubbies give some cycling problems from time to time.

Where did he buy this, and who actually assembled it?
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Old June 8, 2013, 10:16 PM   #5
k511
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He bought it the other day on a whim...only paid 750 for it ..who knows who has assembled it. It appears to be assembled fine ( comparing the internals to my Colt left 6920)...just found out that when he bought it, it was suggested the he add a heavier buffer...could this be the problem?
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Old June 8, 2013, 10:19 PM   #6
Nathan
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I would have said lighter buffer, bigger gas hole or lighter spring.
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Old June 8, 2013, 10:33 PM   #7
k511
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K...I don't understand the whole buffer thing...could someone elaborate?.. ive only owned one AR (colt) and have left it stock ( ain't broke, don't fix it)...I would have assumed a heavier buffer with a smaller hole would trap more gas, allowing the bolt to rack with more force?
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Old June 9, 2013, 02:46 PM   #8
Theohazard
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Quote:
I would have assumed a heavier buffer with a smaller hole would trap more gas, allowing the bolt to rack with more force?
Just the opposite. A smaller gas port hole will allow less gas through and therefore there will be less gas pressure to work the action. And a heavier buffer will slow down the bolt movement and soften the recoil impulse; if it's too heavy it will prevent proper extraction and ejection.

It A 7-inch 5.56 AR-15 has terrible ballistics compared to a longer barrel and is generally a pain to get working right; there is a lot less wiggle room in that small a package if everything isn't tuned just right.

It's hard to tell from your description of the problem if his AR is under-gassed or over-gassed. Does it ever properly eject? If it does, is that brass coming out relatively slowly and ejecting more than 45 degrees to the rear? If so, there's not enough gas pressure to reliably cycle the action. First, have him check the alignment and fit of the gas block. If he takes it off, he'll see a ring of carbon around the gas port in the barrel; if the gas port is completely inside that ring the gas block is aligned properly. Next, have him try a lighter buffer and/or spring; that's the most common fix for an under-gassed rifle.

However, ejection problems can also be due to the rifle being over-gassed. With too much gas pressure, the rifle will already be extracting the fired case while the pressure is high and the case will be ripped from the chamber while it's still expanded against the chamber walls. This can cause everything from a little flash out of the ejection port to chewed-up brass and failures to extract. When the rifle does properly function, it will throw brass violently straight sideways or even forward. The easiest fix for an over-gassed AR is a heavier buffer and sometimes a heavier spring too.
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Old June 9, 2013, 03:30 PM   #9
k511
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Thanks for that info..I'll check that out. .I did get it to cycle some heavier steel core loads yesterday, they were ejecting slowly
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Old June 9, 2013, 11:30 PM   #10
k511
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Could someone recommend a buffer specifically for a pistol? I'm having trouble locating any that do not say carbine
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