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Thread: 300 AAC built upper issue FTE/FTF

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    300 AAC built upper issue FTE/FTF

    Finely got time to take my AAC built upper 300 out today, I will not fire more than one or two rounds before a FTF, or a FTE. I'm using a both Remington 220 gr OTM subsonic, and CMMG 175 grain subsonic both ammo have the same issues, my setup is a AAC 16" and I'm running a 762SD suppressor. I'm not sure whats the exact cause, but it doesnt want to feed out of the mag, has a hard time picking up a new round, out of the mag when its being charged. Then sometimes it requires a recock it to get it to fire then if it does eject it will either work as it should or the case will get stuck sideways, causing a jam. If it does manage to eject, it will fail to pick up the next round. First time I have ever had these issues with any caliber AR, I did notice it was very stiff and requires some "force" to get it to lock up, empty even. Does it need more break in, different bolt, buffer spring? Not sure where to start on this one, but if it needs a break in, it will take a while, at this rate.
    Thanks
    Last edited by mstennes; 02-12-12 at 02:17.

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    Quote Originally Posted by mstennes View Post
    Finely got time to take my AAC built upper 300 out today, I will not fire more than one or two rounds before a FTF, or a FTE. I'm using a both Remington 220 gr OTM subsonic, and CMMG 175 grain subsonic both ammo have the same issues, my setup is a AAC 16" and I'm running a 762SD suppressor. I'm not sure whats the exact cause, but it doesnt want to feed out of the mag, has a hard time picking up a new round, out of the mag when its being charged. Then sometimes it requires a recock it to get it to fire then if it does eject it will either work as it should or the case will get stuck sideways, causing a jam. If it does manage to eject, it will fail to pick up the next round. First time I have ever had these issues with any caliber AR, I did notice it was very stiff and requires some "force" to get it to lock up, empty even. Does it need more break in, different bolt, buffer spring? Not sure where to start on this one, but if it needs a break in, it will take a while, at this rate.
    Thanks
    Sounds like it could be short stroking. What magazines are you using? What buffer and what Buffer spring? Dose it pick up a round every time you manually charge it with no problem? Is the bolt group well lubed? Sorry about all the questions but we need this info to get down to your problem.

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    No idea but I would start with a different buffer and spring. If not them a different bcg. Process of elimination and narrow it down.

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    When you say lock up, is this empty? If so, I would first clean it well, inspect and lube, work the charge handle and release from full compression(empty) and if it's not reliably cycling and locking up unassisted, you need to contact AAC. Otherwise, yes, sounds like a gas issue to me. I would check mags as well, try others.

    This is a tech question, and I will move it over there for some of the Smiths and gun plumbers.--it's still an AR
    GET IN YOUR BUBBLE!

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    Quote Originally Posted by cabbynate View Post
    Sounds like it could be short stroking. What magazines are you using? What buffer and what Buffer spring? Dose it pick up a round every time you manually charge it with no problem? Is the bolt group well lubed? Sorry about all the questions but we need this info to get down to your problem.
    No problem, USGI 30's and a 20's. Magpul 30, I have tried maybe 7 or 8 different mags, bolt and carrier dripping with Break Free, No it does not pick up rounds very well at all, no real consistancy. Does not matter ifs under gas pressure or being manually charged. GI buffer spring H1 buffer. Even when its being operated manually, empty (no round) it still seems reel tight going into lock. When it does fire the empty case seems to get caught in the upper, sometimes it just wont extact, and tries to jam a loaded round into a empty case. About the only thing is hasnt done is double feed.
    Last edited by mstennes; 02-12-12 at 09:55.

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    Quote Originally Posted by mark5pt56 View Post
    When you say lock up, is this empty? If so, I would first clean it well, inspect and lube, work the charge handle and release from full compression(empty) and if it's not reliably cycling and locking up unassisted, you need to contact AAC. Otherwise, yes, sounds like a gas issue to me. I would check mags as well, try others.

    This is a tech question, and I will move it over there for some of the Smiths and gun plumbers.--it's still an AR
    Sorry and thanks.

  7. #7
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    Try that upper on a known working lower.
    Chief Armorer for Elite Shooting Sports in Manassas VA
    Chief Armorer for Corp Arms (FFL 07-08/SOT 02)

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    I wish I had it to look at - it is the kind of thing I could instantly debug.

    When it is empty - and with a known good spring and with lube, if you pull the charging handle back an inch, and let go - will the bolt lock into the barrel extension? If yes, then that is fine. If no, then the bolt, barrel extension, carrier, upper, or cam-pin is bad. The easiest thing to try is another carrier assembly from one of your 5.56mm rifles and see if that changes it. If it does, then change one part at a time to isolate. If it does not, then if the carrier rubs in the upper, it is the upper. If the carrier does not rub in the upper, then it is the barrel extension.

    You can also send it back to AAC - but I am trying to save you time.
    Last edited by rsilvers; 02-12-12 at 11:00.

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    Quote Originally Posted by rsilvers View Post
    I wish I had it to look at - it is the kind of thing I could instantly debug.

    When it is empty - and with a known good spring and with lube, if you pull the charging handle back an inch, and let go - will the bolt lock into the barrel extension? If yes, then that is fine. If no, then the bolt, barrel extension, carrier, upper, or cam-pin is bad. The easiest thing to try is another carrier assembly from one of your 5.56mm rifles and see if that changes it. If it does, then change one part at a time to isolate. If it does not, then if the carrier rubs in the upper, it is the upper. If the carrier does not rub in the upper, then it is the barrel extension.

    You can also send it back to AAC - but I am trying to save you time.
    Well I just "borrowed" a BCM bolt carrier, and bolt out of one of my known working 556's, and it ran 5 rounds no problem through one of my 20's, I used the one that didnt work worth a damn before. It now closes just fine, like any other AR I have ever dealth with, I did try it on the 556 doner rifle, and the problem followed, so something with the AAC bolt carrier group is not just kosher, I'm going to try it in different AR's of mine today, and see if its actually the bolt or what?

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    Quote Originally Posted by mstennes View Post
    Well I just "borrowed" a BCM bolt carrier, and bolt out of one of my known working 556's, and it ran 5 rounds no problem through one of my 20's, I used the one that didnt work worth a damn before. It now closes just fine, like any other AR I have ever dealth with, I did try it on the 556 doner rifle, and the problem followed, so something with the AAC bolt carrier group is not just kosher, I'm going to try it in different AR's of mine today, and see if its actually the bolt or what?
    You may want to take apart the BCG and inspect the rings...

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