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Old 02-25-2013, 10:26 PM
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Question ? about AR-15 Short Stroking

Hey guys I know this isn't a Bushmaster Forum but I'm a Smith guy that's why I'm here. I went shooting with a friend today I took out my new shield and he took his brand new Bushmaster Carbon 15. Yes I know this gun is not a good investment but he got it on black Friday at Cabelas before I could tell him not to buy it. He loaded it up 30 round mag that came with it. First round bang then next round click. It wasn't even picking up the next round out of the mag at all but bolt was closing all the way back closed fully. So pulled the bolt back manually and it chambered round no problem then bang again then click same thing not picking up 2nd round. I had him try a new 30 round P-mag and same results. So here is what we did. We swapped out the complete bolt carrier assembly with another AR-15 and it functions properly with any magazine. So what could be the problem? Do any of you have any ideas? We did tear it apart when we got home and clean it up and the bolt did not seem to freely rotate and go in and out of the bolt carrier with ease like it should like it had some resistance. We took it apart and polished it up clean then lubed. The big pin that you have to rotate to pull out of the bolt so that you can remove the bolt from the carrier seemed like it was to snug also. We couldn't even get it back into the bolt when installed back into the carrier it was so tight. We polished it up and finally got it to go in. Got it put all back together and the bolt then seemed to rotate and pull in and out of the carrier as it should. So getting the gun all back together it made no difference it still wont pick up the 2nd round anytime and if you just chamber 1 round and shoot it bolt doesn't stay open when the mag is empty. This thing seems to be short stroking and its got us stumped. The gas keys dont seem to be properly staked either but the hex screws are very tight and not loose. They are staked so lightly that they could easily back out over time.

Again this gun is brand new first trip out to line in the red dot. If you have any ideas please respond. Hes contacted Bushmaster and they said they have no parts for replacement or to fix and have no idea when they would have them. My friend is not a happy camper.
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Old 02-25-2013, 10:36 PM
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If Bushmaster can't help, I suggest taking it to a reputable gunsmith and have them look it over.
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Old 02-25-2013, 10:40 PM
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It worked with another carrier and bolt, but not with the OEM? Hmmm. If it didn't work with either I'd check the buffer/recoil spring, but since it works with one and not the other, I don't know. Are there marks from burrs on the inside of the upper, or burrs on the carrier that might be causing it to drag? That's all I can think of, since it works with another carrier/bolt.
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Old 02-25-2013, 10:46 PM
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Yes it worked perfect with different bolt and bolt carrier swapped quickly from another AR-15 just to see if it made a difference. We cant find anything wrong with the OEM bolt or carrier but it just will not function properly with it. It will fire but will not pick up the 2nd round and put it in the chamber with the OEM bolt assembly. It is ejecting the spent case but not picking up the 2nd round like its not going back far enough to catch the 2nd round but far enough to eject the hull.
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Old 02-25-2013, 10:51 PM
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Did you put the carrier/bolt from the Carbon into the other AR and test fire it? If that combo works, I don't know what it could be. If it doesn't you might want to see if the OEM carrier is warped or out of spec. That's all I can think to do.
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Old 02-25-2013, 10:58 PM
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Quote:
Originally Posted by Mike7.62 View Post
Did you put the carrier/bolt from the Carbon into the other AR and test fire it? If that combo works, I don't know what it could be. If it doesn't you might want to see if the OEM carrier is warped or out of spec. That's all I can think to do.
No we didn't try that. But I do know we compared the bolt assembly's and the OEM was a bit shorter than the assembly from the other AR-15.
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Old 02-25-2013, 11:06 PM
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First, take the bolt out of the carrier and make sure the gaps in the gas rings aren't lined up. If they aren't, start swapping the parts with the bolt and carrier that did work until you isolate the problem. Could be the cam pin, bolt, firing pin retainer, or the bolt carrier.
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Old 02-25-2013, 11:11 PM
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Quote:
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First, take the bolt out of the carrier and make sure the gaps in the gas rings aren't lined up. If they aren't, start swapping the parts with the bolt and carrier that did work until you isolate the problem. Could be the cam pin, bolt, firing pin retainer, or the bolt carrier.
Ok I will tell him to do this tomorrow. I'm sure hes in the bed right now. So does the length make a difference on the bolt carrier itself? I thought they were the same but apparently im wrong.
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Old 02-25-2013, 11:25 PM
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They should be the same length. Since this rifle worked with the other bolt and carrier, you can rule out the buffer and spring, the hammer, and the gas system. Something in the original bolt and carrier is probably out of spec or burred. The gas rings would be the easiest to check.
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Old 02-25-2013, 11:33 PM
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Quote:
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They should be the same length. Since this rifle worked with the other bolt and carrier, you can rule out the buffer and spring, the hammer, and the gas system. Something in the original bolt and carrier is probably out of spec or burred. The gas rings would be the easiest to check.
Well the OEM assembly is a bit shorter than the other assembly we got out of the other AR. I dunno exactly how much shorter but very noticeable.
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Old 02-25-2013, 11:39 PM
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I done some reading using Google and seems the Carbons do use a shorter bolt assembly but why did the normal one work flawlessly in it? I'm stumped.
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Old 02-25-2013, 11:41 PM
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Shorter where? In the back or the front? Is it shorter in the front (which makes the gas sit key back further)?

If you put the two carriers next to each other and lined up in the front, do the gas keys look to extend to the front the same distance, or is this where you notice the difference?

This is my theory. The gas tube is seated deeper in one gas key than the other, when they are fully forward into battery. The one that is back further - is losing pressure - not driving the bolt carrier back.

Last edited by gm272gs; 02-25-2013 at 11:44 PM.
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Old 02-25-2013, 11:47 PM
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Compare the two bolt carriers. I bet the one that works is a "full circle" and the one that came with the gun isn't. It may be counter-intuitive, but a heavier bolt carrier will retard the bolt opening, so the chamber pressure is lower when the bolt starts to open. High pressure keeps the brass tight against the chamber walls, and the result is that the action is retarded. Sometimes you'll rip the rims off the cases with this situation, too.

Let me find a photo of what I am talking about....here ya go:



hth
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Old 02-26-2013, 12:06 AM
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I just pulled and measured three bolt carriers, one is a 30 year old EGW similar to the top carrier in 2hawk's picture. The second is a Colt half circle, the third, a M16 configured carrier like the bottom example in 2hawk's picture. All three measured 6 5/8 inches. If your friend's carrier is longer, it may well be the problem.
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Old 02-26-2013, 10:02 AM
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I will try to get a pic and post it. Thanks for all you guys info and help.
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Old 02-26-2013, 10:31 AM
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Have you tried AR15.Com?
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Old 02-26-2013, 12:21 PM
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Ok here is the best pic I got that he sent me. The carriers are the same size the only difference it the opening size you see here in the pic. There identical otherwise except the poor staking on the bushmaster. The one with the pen on it is the other bolt carrier the one in the front is the bushmaster.
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Old 02-26-2013, 12:26 PM
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Quote:
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Have you tried AR15.Com?
No sir I haven't. I wanted to try here first because I know I wont usually get bashed.
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Old 02-26-2013, 12:30 PM
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There you go. Heavier BCG works better. If your buddy doesn't want to change out the BCG he can go for a heavier recoil spring and/or a heavier buffer and see if that will work. It is a balancing act between the amount of gas driving the action, the weight of the BCG, buffer weight and spring. I'd suggest a heavier buffer as the best easy fix, changing the spring rates seem to add a factor of ammo sensitivity.
HTH

ETA arfcom is the place for AR knowledge. You can find tons of good information in the stickies in all of the technical forums. Just stay our of the general discussion area, where no amount of idiocy is too much.
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Old 02-26-2013, 12:32 PM
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I thought new guns had a warranty.
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Old 02-26-2013, 12:35 PM
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Quote:
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There you go. Heavier BCG works better. If your buddy doesn't want to change out the BCG he can go for a heavier recoil spring and/or a heavier buffer and see if that will work. It is a balancing act between the amount of gas driving the action, the weight of the BCG, buffer weight and spring. I'd suggest a heavier buffer as the best easy fix, changing the spring rates seem to add a factor of ammo sensitivity.
HTH
But why would Bushmaster send out this gun if it isn't gonna work properly? This was first this gun has been fired he shouldn't have to be working on it.
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Old 02-26-2013, 12:37 PM
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This is why I buy Smith & Wesson. I have the 2012 shot show magpul edition AR and love it. I tried to get him to get the sport cause it fit his budget and he wishes he would have now.
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Old 02-26-2013, 12:40 PM
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What ammo is he using? I suspect you guys were shooting M193 or something with a a similar power level. They build a lot of guns to cycle on cheaper ammo, so the guns tend to be overgassed. They can't account for all variables at the factory, not to excuse them. He should call CS describe the problem and ask if they can send a heavy buffer. Beats sending a gun back, it's a $10 part and a 1 minute fix.
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Old 02-26-2013, 12:43 PM
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One other thing that just came to mind: make sure the chamber is really clean, and free of machining or burrs. If you need to, get a chamber brush and a section of rod, chuck it in a cordless, and get in there with some CLP for a minute or two. If the gun does have a bad chamber it probably should go back under warranty.
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Old 02-26-2013, 12:58 PM
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Quote:
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I thought new guns had a warranty.
Yes sir they do was trying to figure out the cause to see if its just something simple instead of being without a gun for weeks. Plus bushmaster told them they didn't even have any parts in stock to fix it.

As far as ammo we were using Tulammo FMJ 55gr and it wouldnt even blow the bolt open and stay open on last round.
Seemed to work a little better with the American Eagle 55gr FMJ but still same issue.
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Old 03-03-2013, 06:19 PM
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He got a new buffer spring and it fixed the problem. Just a update. Thanks for all your help.
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Old 03-05-2013, 08:45 AM
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Inspect the gas rings on the bolt carrier.. make sure they are well staggered and there gap is not lined up.. Buffer spring check. make sure gas key is clear and also check the gas tube make sure it's clear and installed right in the gas block.. Clean all grease out of the gun and for the first 100 to 200 rounds run her wet with Rem oil with Teflon..Oh and IMO don't use steel case ammo! George
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Old 03-19-2013, 06:31 PM
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What kind of ammo were you using? Cheap 223 steel cased? It sounds like the BCG just needs to be broken in. I would clean and lube the BCG properly first. Then hand cycle the carrier with the charging handle 100+ times to let the bolt and carrier bed itself into the upper smooth out some of the surfaces that rub and rotate against other parts. Then when you take it out, shoot so equality 556 rounds through it as it will have more power to push the BCG all the way back so it'll strip a round off the mag as it should. You can also shoot one round per mag to see if the carrier is held ack on the last round. If it does, them you're good to go.
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