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Posted: 12/29/2006 4:24:38 PM EDT
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What are the causes of failure to lock in a semiautomatic M4orgery? The round was picked up from the magazine, started up the feed ramp and into the chamber, but the bolt/bolt carrie didn't finish the last 3/8" of travel and the bolt did not lock. There is unusual wear on the bolt face side on two of the locking lugs. Also, odd and excessive wear pattern on the bolt cam pin based on the number of rounds fired (less than 20 total). Gas key showed no signs of leaking around gas key when checked under 10X magnification. Bolt seems difficult to open when weapon empty and charging handle pulled back. New weapon (100% Stag Arms, upper built by Stag Arms, lower assembled by myself and function tested - passed without incident), complete weapon cleaned with Ed's Red, then lubed with Breakfree/CLP for break in. Carrier gas key checked, retorqued and restaked before initial firing. Fired six rounds before experiencing failure to lock. Cleaned rifle thoroughly, relubed with a generous amount of Breakfree/CLP - problem reoccured - another failure to lock after 6 rounds. Tried other known good magazines - problem reoccured. Known good 20 round straight magazine, Colt label. Ammo was M855 ball manufactured in 1991. Ammo came from sealed ammo can, was still in original bandoleers. Tried ammo and magazine in known good AR15A2. Ammo fired beautifully and accurately for 60 rounds without incident. |
| Dave, anothr Dave on this end. It sounds as though the bolt and the barrel extension aren't quite lining up just right. The bolt when closing the action should move free and smooth into battery. You are describing some form of contact (as exhibited by the wear) which you mention. I think it is all in your upper. Not the ammo, magazine or lower. I would try to see how they mate with the upper taken off the lower and inverted to see the bolt go home into the barrel extension. |
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Try the BCG test mentioned here many times . Seperate the upper from the lower , Now pull the BCG back all the way and with the upper point down at 45 deg. the bgc should slide into battery when released. (leave the charging handle forward ) Should also be able to retract the bcg group by just sticking your finger in back of the bc. If this fails something is amiss , check gas tube for alingment with key or as mentioned how the bolt mates with the barrel ext. Maybe the cam pin is binding with in the carrier (should move freely) |
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Gentlemen, I tried out the methodology both of you suggested. It's definately not the gas tube/carrier key alignment, but it does appear the bolt isn't quite right and doesn't quite align with the barrel extension. Because the upper is brand new and was assembled by Stag, I made the decision to return it to them to sort out under warranty, as I don't have tools or parts. Seemed the simplest thing to do. Thank you for the suggestions. I was able to see enough to feel comfortable it was the upper that was the problem. Regards, Dave |
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