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Posted: 5/13/2005 6:12:18 AM EDT
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Everything is new except the barrel, which is like new from Fulton Armory. It ejects every time but about 50% of the time it does not come back far enough to pick up the next round or lock open on an empty mag. I've tried different mags and am shooting full power military stuff (M855 and Mk262). The carrier key is snug, no looseness. Again, it doesn't do it every time, just about half the time. Could it be that it just needs "shot in", i.e. maybe the gas rings are a little tight fitting as everything is new? I only have about 40 rounds through it (got kinda tired of manually chambering rounds). I had a shorty some years ago that had teething problems and was built with all new parts. It took about 150 rds before it functioned smoothly and then it ran like a champ. |
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If its all new parts, I would first lube up the upper receiver, and the bolt carrier group. If it still is short stroking then I would worry about parts that need to be looked at, replaced, or repaired. Usually the easiest solution is the one that works. I had the same problem as you with a new carbine, and it ended up being not lubed enough and a rough receiver extension that was slowing and catching the buffer. After a few hundred rounds the problem fixed itself just from wear inside the receiver extension. |
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The gas needs to come out of the barrel and into the tube without obstructions. It needs to get into the bolt carrier without leaking from between the key and the carrier. The carrier needs to be able to slide smoothly in the upper. The bolt needs to move smoothly in the carrier. The barrel extension needs to be free of burrs and gunk that would slow down unlocking and waste gas and impulse, thus slowing the action enough to keep the bolt from going all the way back. I think I covered all of it. Other people have posted various specific issues that impact what I say, but it's all the same thing. With a "practically new" barrel you have to definitely think about FSB alignment and barrel extension problems, but the rest of it needs attention too. |
Be sure to lightly lube the action spring with CLP. Easiest way to figure this out is to use a known good carrier in this rifle. Fire 60 rounds. If the short stroking stops, you may need a new gas key, or a better quality carrier. |
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I had the same sort of problems with mine, as quite a few others seem to have done also. I can tell you what worked for me, and similar things *seem* to have worked for at least some others: I can't be absolutely specific about what made the difference, because I changed two things at once... Check the obvious stuff first - that the bolt is properly assembled, gas rings spaced at 120 degrees, key tight and firmly fixed, gas-tube aligned so that it does not move *at all* as it engages the key, no dents/crimps in the gas tube, gas tube correctly inserted into the FSB. Assuming all that is correct, what I noticed on mine was that with the bolt not engaged in the barrel extension (ie. not locked), if I held the upper vertically the bolt carrier did not slide right out. It moved a little way and then stopped. it was relatively easy to pull out, but there was an obvious difference between the first couple of inches and the rest of the travel. I had lubricated everything with CLP. I cleaned off the CLP on the bolt, carrier and inside of the upper, and applied Miltec-1 to everything, exactly as described in the instructions on their website - very thin coating, heat with a hairdryer until the metal is hot. When reassembled, the difference was noticable - the bolt carrier would now happily slide out when the upper was held vertically. I had two buffer springs, so I compared the installed one against the other - there was about 1/2" difference in length - so I removed the longer one, and installed the shorter one. Back to the range, and it cycled quite happily, and locked back every time that the last round was fired. It has continued to work flawlessly since. My *guess* is that it was tight because it was new, and just making those minor changes was enough to get it going. I suspect that I could go back to CLP on everything and put the orignal spring back in now and it would work just fine. The mechanical parts are now bedded in, high spots worn down etc. The gas hole in the barrel actually gets bigger quite quickly during the first thousand rounds or so, so gas flow improves, whjich also helps. So, unless there is some really obvious mechanical problem, just do what you can to make it easy to cycle, and fire a few hundred rounds or so before starting down the road of pulling FSBs... BTW: I have a Fulton Armory barrel on my 20" A2, and it works just fine. The M4gery is a DPMS barrel. |
Or he may need to properly lube his new bolt/carrier if there was not enough lube used in assembly. |
| I figured you guys would have a few suggestions. I'm gonna strip her down and lube it up while checking a few of the things mentioned here. My suspicion is that with proper lubing during break-in (assuming no other obvious problems) that a few hundred rounds might fix it. You don't need a bigger hammer, just more whacks! |
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Just run it really "juicy" for a few hundred more rounds ABNAK. If it's still short-stroking (are you positive that's what's happening?), my first guess would be a gas problem... like everyone already said. Just for the hell of it, before you pull everything apart, remove the buffer-spring and pull CH back slowly. Does it feel right? Edited to add: If you do remove the buffer spring, take a peek inside the buffer-tube. There are no scratches from the BC, right? 
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