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Posted: 11/27/2007 5:49:33 PM EDT
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I'm having problems with a used rifle that I just purchased. Here are the specs and symptoms: Rock River Varmint 20'' (seems to have had little use) Remington 55grn UMC Ammo The gun is failing to load a round. After firing a shot, the second trigger pull will result in a misfire, or rather, an empty chamber. The bolt has to them be manually cycled, wherein, the same problem repeats itself after the loaded round it fired. It seems there is sufficient pressure to open the bolt far enough to extract the round fired, but not enough for the bolt to pick up a new one. At first I suspected the mags but after using several types, I'm having the same issue. My first thought is to take the gas tube apart or try using different ammo. Before I go there, does anyone have any other ideas or previous experience leading to the cause of this problem? |
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A good game plan would be to check the gas block to confirm it's positioned correctly with the barrel gas port (read pull the block and confirm that the burn spot is centered around the barrel port) and check the carrier key to confirm that it's still tight. From there a quick check on the back of the disco for signs of hammer wedging, followed up by a quick check of the recoil spring to confirm that the standard spring has not been replaced with a extra power spring and causing problems. Also, the rifle should be ejecting the spent brass from 10 to 12 feet. If you are getting this (read rifle cleaned and lubed correctly), then the problem could be all mag related (mag or just the rifle holding the mag low with a loose mag catch. Or, something in the receiver extension limiting the amount that the buffer can move reward, and the bolt never reaching the back of the round to strip it cleanly (or at all) from the mag. Note: the mag catch threaded section should be flush with the face of the button. |
| Try some military 5.56. Winchester or IMI. A lot of times, ARs will short stroke (what yours is obviously doing) when new. After about 500 rounds they tend to loosen up. Most of the time I think it is from a tight bolt to carrier fit. That is what caused my problem on my Stag. The gas rings were very tight, causing too much friction, even with lots of lube. It works with full pressure military stuff, but not with the .223 light loaded stuff. Another cause is the buffer spring sometimes is out of spec and too stiff when new. I was aggravated when I bought mine previously owned but never fired and it was doing this. I checked everything out and the only thing I could come up with was the bolt to carrier fit, so I swapped with a Bushmaster that is smooth and had about 600 rounds through it. That solved the problem. Unfortunately, I have not had the opportunity to get 4-500 rounds downrange, I seem to be stuck at about 70. But, I did hand polish the internals of the carrier with highly fine metal polish, and the externals of the bolt and got it working with everything except Wolf, and it sometimes cycles Wolf, which it never did before. The Wolf I have is military classis and they sent me shipping labels to return it, but I am convinced that I can get it going if I can get some range time to break it in properly. Besides, my Bushmaster runs fine on it and I would rather have the ammo that not. Also make sure your chamber is clean when new. It will also be on the tight side and uses some of the bolt inertia to extract the spent shell, so lowering the force required to initiate extraction will improve cycling. |
| Thanks for your responses. I did notice one thing while I was cleaning the rifle. Is the gas tube supposed to be loose in the receiver end? I can move it around a little (1-2mm) with my finger or a Q-tip. Assuming it's not supposed to be that way, is there something that should be seating it where it comes in from the float tube? |
The gas tube should be semi loose and able to be moved in the upper receiver. This movement is helps the tube to align as the carrier (key) moves towards it (read the B/C does not ride tight in the upper, so the tube needs to flex a bit to allow for such). |
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From Dano's post: What's the disco? I am fairly ignorant about AR's. I've build a lower and know how fieldstrip them but that's about it. The burn spot on the gas block is fine and the spring looks to be within specs. I also swapped the bolt and carrier from another AR for a couple rounds but the the same story. I'm writing because I finally got to take the rifle out again. This time I had three different kinds of ammo: M855 62gr - Rem UMC 55gr - Silver Bear 62gr. The Rock River is an ammo snob. It only liked the expensive M855. The same failure to load or fully eject happened with all others. All of the ammo worked fine in the other AR. The problem is that the M855 is that it's expensive and I wanted to save what I've got. What can I do in order to use cheaper ammo? |
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Disconnector, and its the part that holds the hammer back until you release the trigger. As for being ammo selective, give the rifle a chance to break in and you will find that it will run with the less powerful ammo (read right now, the upper/rings are still a little grabby/bindy and causing the cycling problems). |
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Have you checked the fit between the upper and lower? RRA is putting a bunch of out of spec lowers out recently. See my thread. I had mag issues and occasional FTF and FTL. www.ar15.com/forums/topic.html?b=3&f=66&t=358529 |
| yeah, the first thing that i would check is the buffer spring, i was having this same problem with my ar and i switched the buffer spring out with a weaker one and it worked just fine. something to try, its easier to do than disassembling the bolt and carrier again and again |
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