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Posted: 7/8/2008 1:28:51 PM EDT
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About a month ago I got my second AR15 and now have taken it out twice with it not working right. What is happening is the majority of time one round will fire and the brass will be ejected but a second round either isn't picked up or the bolt doesn't fully close, with a round not being picked up happening much more often. I'd say less then 5 times it started to strip a round from the magazine and then hung up with round part way up the feed ramps. This is a brand new 16" upper from CMMG on a DPMS stripped lower with a RRA NM 2 stage trigger parts kit and an ACE entry stock. To eliminate the lower I swapped uppers between my two lowers and the 20" upper worked just fine on the new lower but the new upper didn't work on the old lower. We also pulled the recoil buffer and spring from my lower and compared to a friend's. His spring was between a 1/2" and 3/4" shorter but he had several thousand rounds on it. Just in case we did put his buffer and spring into my lower with no improvement. I've tried multiple different magazines including 30 round GI, 30 round PMags and a Colt 20 round. All magazines show the same problem. Nearly all the ammo I've tried is XMAE223 63gr SP which works fine in my other AR15. I fired a handful of rounds from a friend using his Colt 20 round and had the same problem. We looked closely at the gas key and it is tight. The staking job did create metal to metal contact with the screws. I also used a bore light and looked down the barrel and didn't see any burrs or obstructions. I took a picture of the gas key to show what it looks like. http://www.geoffrion.org/ar15/key.jpg From a purely subjective point of view it seems like it takes more effort to pull the charging handle all the way back on the new AR. With the snapcaps I have I was able to get the bolt to hang and took this picture. http://www.geoffrion.org/ar15/bolt.jpg I'm sending this same info off to CMMG since it is a new upper but I'd love to hear any possible solutions or things to check. CMMG 16” M10R Upper A3 Semi-Auto Carrier No Bolt Group Upgrade Badger Ordnance Gen II Tac-Latch Extended/Polished Ramps A2 Post F Marked Front Sight Base Round Forward Assist YHM Light Weight Rifle Length Free Float Four Rail Handguard YHM Solid Rear Sight YHM-9493S Phantom Brake 5M2 |
| First try swapping the bolt carrier out and see if you have the same problem. If you do I'd check the gas tube for signs of contact such as chipping around the ends. If you don't seen anything put the original bolt carrier back in with the charging handle, pull the bolt carrier back about two inches lock the charging handle in place and slowly push the back of the bolt carrier forward, it should go forward and lock with little resistance. If you find some resistance you have contact between something if you can locate it and smooth it out it should be fine. If all that checks good and it does this with several diffrent types of ammo, you might be looking at an out of Spec upper. |
I pushed the bolts in on both and they seemed to take the same amount of effort no matter which upper they are in and have just the slightest bit of resistance and then click into place. I'll have to get back out to the range this weekend and fire some ammo through, really wish I had thought to swap BCG's while I was at a range. |
| Made it out to the range today. With the bolt group from my other AR15 it was better, but still not perfect. I did 4 - 3 round magazines to work on sighting in and they all went right except on the very first magazine the bolt didn't hold open. I then loaded up a 30 round magazine and on slow aimed fire it didn't pick up the next round twice. |
| After you changed out the BCG from waht you said it now locks up alright but is still not traveling properly. Did you see any signs of excessive wear inside the upper receiver, or any sign of venting gas outside the normal? If not it still could be an out of Spec upper, but I would take the old BCG you had that worked lube it very heavy and put about 100 rounds thru it. Then inspect the upper again for signs of excessive wear, if everythng looks good try the original BCG again. If it still does not lock try to replace just the bolt this time and see what happens. |
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