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Posted: 2/10/2007 10:41:59 AM EDT
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I've got a problem recently with my CMMG M4gery upper. I have never had a single issue with it before. I shoot at an outdoor range. Lately temperatures have been around or below 20 degrees. Today it started snowing a bit while shooting. I clean after every range session and lube it up like I always do -- with a generous helping of Mobil 1. I shoot Wolf ammo and my own hot handloads. The issue happens more with Wolf, but happens with either load. The upper has been uber-reliable and has about 3200 rounds through it. It has not been altered in any way other than to add optics, etc. The bolt is original with no change to it (until the last range session I had). It was having trouble where it would fail to feed or lock back the mag on an empty mag. The feeding trouble would be that it wouldn't feed a cartridge at all. It would eject, but it wouldn't go back far enough to feed the next one. No jam though. I would just manually pull the bolt back and I'd be good to go. This would happen only occasionally but would increase in frequency as the range session went on. This happened the last two range sessions, so I decided to replace the springs on my range mag. The problem still did not go away. Today at the range I didn't have any trouble at first and then the issue became more and more prevalent till at the very end of the range session I would have to hand cycle every time. In fact, it would not even cycle far enough back to eject the spent casing. In the middle of the range session I replaced the gas rings on my bolt. No change. I think swapped the bolt for a brand new spare I had. No change. Now what's going on? Is the issue the cold and my generous helping of Mobil 1?? I've never had any trouble before, but I don't know if I've ever shot it in this kind of weather before either. HELP! |
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I replaced the rings and also ran a brand new bolt. Definitely not that. How could I check on the gas key? I'm going to lightly lube it and run it tomorrow and see how that works. I will admit that when I got home and disassembled it, it was frigid to the touch, so it definitely got REAL cold. |
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Check the suggestions listed above. I will say I had an LMT MRP that did alot of short stroking that I never resolved the issue. I finally sent it back to LMT and they quickly returned a new one, so I never figured it out. The new one has never short stroked in 3k with the same ammo, mags, BCG, and barrel/gas tube-block, so I assume it was an issue with the upper. I wish I knew though what caused it. Good luck. BTW try some know functioning ammo Q3131 or something just to be sure it is not ammo. |
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Excellent suggestions fellas. Keep them coming! I checked the carrier and everything is staked properly still and nice and tight. No play at all. My guess is that it's user-error -- meaning that lots of Mobil 1 in a cold firearm is what did it. We'll find out tomorrow when I go to the range. I don't believe it's an ammo issue. It's never had a problem at all (just checked -- I'm at 3455 rounds according to my log). And it did it with both my hot handloads and with Wolf. And since every time it's done it has been in sub-freezing temperatures and every time it has gotten progressively worse as the range session continued, I'm guessing that it has to be cold and lube related. I'll update tomorrow and let you know. God I love this rifle, so I hope that's all it is. I'm probably going to get a gas-piston upper from POF next as I need a second AR and I think that might not have this problem. |
Regular old aluminum mags. My regular reliable range mags. Oil the gas tube?? Why?? I think the problem is likely that I put A LOT of lube on it normally. I pile it on thick. Which is probably fine above freezing, but the already thick Mobil 1 is probably getting even thicker below freezing. That's what I'm trying to tell myself  . We'll see tomorrow. I applied a light coat of CLP after cleaning tonight and will try again tomorrow.
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I'd also pull the recoil spring and clean the buffer tube and then apply the shean only amount of oil to the spring. Does the spring, buffer, and bolt group feel normal when manually pulled rearward? Is there an excess build up of carbon inside the bolt carrier? Does the bolt move easily in and out of the carrier when that's all you have in your hands? Without ammunition in the rifle, does the bolt close fully without resistance? Keep us posted... |
. I'm heartbroken. My favorite rifle is down. I'm going to contact CMMG and hopefully send it back to them for a fix.
Carbine buffer... I think. Whatever came with it. The action doesn't feel any different than when I first got it, so I don't think the buffer went bad on me.... |
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This MIGHT be totally off the wall, but has anyone considered the different expansion/contraction rates of aluminum and metal or between different metals, affecting tolerances due to temperature? Like MAYBE the cold changed the clearance of the gas tube and key, causing an additional drag? Or other metal to metal contacts of moving parts during cycling? I'm just an Armchair Commando, with NO technical training in the matter, BUT it MIGHT be a possibility.  Lonny |
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one othertime i had similar problems was when i was using an Oly "BlackTac" bolt/carrier. bo matter what i did i couldn't keep it from rusting atthe stem of the bolt and where it went through the carrier. then with those few minute rust pits it would carbon up real fast atthat point. making the whole thing sluggish(this was at about +20F or so). when was the last time you cleaned the carbon out from the base of the bolt hole in the carrier? i asked earlier about oiling your gas tube. some people oil the inside of their gas tube, this is a bad idea. the oil and hot gases make carbon deposits form on th einside of the tube. this decreases gas flow. this may or may not cause probs in warm weather, but prob would in cold. advntrjnky |
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Thanks guys. I got it working again. I gave the gas tube and the area around the gas tube a bath in brake cleaner (like one of the posters above suggested). Good as new. Lots of crap came out and now its working again. I got a Bushy M4 upper as a replacement while this one was done, so the CMMG one will now be my spare. Wouldn't be surprised if it eventually turns into a another rifle... |
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