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Posted: 4/24/2011 3:36:54 PM EDT
All right so here is the deal I have been dealing with a consistent malfunction. Any were between 5 to 15(but not consentient) rounds I experience the same problem, the bolt catches the round and shoves the tip to the top of the fed ramp as pictures. This is a very recent development (last 500 rds) with my rifle that has close to 6k rounds through it (have had the rifle for less than a year). The upper is a BCM 14.5 middy complete and I have experienced the problem with multiple loads of ammo from .223 and 5.56. The spring is a carbine length with a standard buffer. To add another issue the bolt is failing to lock back on an empty mag. My first thought is to put in an H buffer to slow down the cycle rate. It seems that the bolt may be cycling too fast. Any input would be great.
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fouling of the gas tube. Make sure to clean out the key on the bolt carrier. If it gets fouled up with carbon, your carrier is not getting the amount of gas needed for the carrier to go all the way back, causing you rifle to "short stroke" That is what my problem was on one of mine. Caused failure to lock back, and also failure to having next round seated before spent casing ejected. |
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I know you have probably tried this. Try a different BCG , and if that don't cure it try a different Mags.
If the BCG fixes it then try a different bolt in your carrier. If you are having troubles after the BCG swap and Mags swap, look towards a different buffer and spring. My .02 cents worth. Just one more thought, does the barrel and the upper have M4 feedramps? |
| Start with cleaning the top end completly and checking the gas system then make sure your key is tight and your bolts have not worked loose next try a diffrent mag in the gun and see what happens also make sure nothing is stopping the bolt from going all the way back in the carrier. Buffer tube and spring and the tube itself.... |
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Looks/sounds like a short stroke problem to me so as stated check your gas key screws, GB/FSB and also the tube to insure they are all still tight and properly aligned with each other.
Have you completely disassemble the bolt and carrier and cleaned/lubed them both lately?... if not then you should. Also have you cleaned the chamber with a chamber brush using a good solvent and then lubed it lately?... a dirty/dry chamber can slow things down more than you think especially if you shoot any of that coated ammo. There's also a possibility it could be your mags follower is not recovering quick enough sometimes to reliably chamber the next round and trip the bolt catch, have you disassembled and cleaned/lubed the mags lately or tried other mags? Since this started in the last 500 rounds and it functioned fine before I would say while your checking the gas system parts clean/lube the chamber, BCG and mags as stated above and try again. |
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Failure to lock back on an empty mag + funky feed angle screams mag problem. Does it occur with the same frequency over an assortment of USGI aluminum & PMags?
The 6,000 round figure would put the rifle in the ballpark where the length of the buffer spring should be checked (should measure between 10 1/16 inches and 11 1/4 inches). Have also heard of armorers using an H buffer to compensate for gas port erosion on M4s with a heavier than average firing schedule. |
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Problem is either the bolt is not coming back far enough to get past the rim of the ammo in the mag, or the mag is not recovering the top round all the way back up, and when the bolt goes forward to strip the round, it grabbing the round out of the mag via the middle of the case, and not behind the rim.
Clean and lubed rifle, single loaded round into the mag, and it charged/fired for effect. The bolt should lock back on the catch with the ejection distance around 10' to 15' out of the spent case. If this not happening, double check for the key bolts to be loose (torqued at 37inlbs), or one of the bolts snapped. If the bolt locks back/ the ejection distance is correct, then suspect that the mags are not recovering correctly, and just pulling the mags down to CLP clean them may solve the problem with them. |
| I've spent the day working through every variable. I used different lower, bolt, BCG ect. and found that it is something to do with the upper specifically the gas system. I've already cleaned it once but I’ll do it again and see it has an effect. At this point I’ve contacted BCM being that is a complete upper from them and see if they can help. |
| Take a look at the gas tube mushroom. Sometimes a worn gas tube will cause a gas leak which might be the issue. I have to agree with the other folks, sounds like it needs more gas. If you have the tools, you might want to pull it apart and check the gas port/block. Might have an obstruction. |
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Quoted: Take a look at the gas tube mushroom. Sometimes a worn gas tube will cause a gas leak which might be the issue. I have to agree with the other folks, sounds like it needs more gas. If you have the tools, you might want to pull it apart and check the gas port/block. Might have an obstruction. This, if gas key screws, gas rings and fouling in general of BCG have been ruled out - it's all clean and lubed up good right? Carbon loves to buildup in / around gas port and that's a small hole. I'd pop the gas tube off too, there is plenty of space for carbon to buildup there, inspect tube for cracks and check the o-ring. Soaking in Slip 2k Carbon Killer and blasting with compressed air might help if you lack tools to break it all down. Doubtful there is a burr there after so many rounds, but that can happen too. Don't change buffer / spring until ensuring that gas port / tube are GTG. |
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