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Posted: 9/1/2013 3:33:48 PM EDT
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Went to the range today with a built upper that has never been shot. Equipment is a rainier upper, sig mp400 14.5" barrel, stock mp15 BCG, and a lower profile gas block. Stock mp15 lower. When I was able to get a shot off the brass ejected fine (or enough to not get stuck in the chamber) but after one or two shots it would fail to chamber the next round After I chamber the round manually it will shoot that round fine but then the bolt would go half way and not chamber the next round fully. I suspected a gas issue so I took the gas tube off and checked to see if it was crimped in anyway which it was not. Then checked the alignment of the gas block via the paper clip method and it clears into the hole in the barrel. I haven't tried compressed air into the gas tube yet but it seems to clear the other side when I blow into it by mouth. I can't seem to figure this out.
My BCG worked with my old upper and I haven't touched it since I shot it last. Could something have gone wrong with the BCG not cleaning it the last time I shot? I hope its not that fragile. I did the stand the bcg on the bolt thing and it stayed up. I did also notice my brass deflector had some brass colored markings on it which not sure if thats normal or not. |
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What is the load of the round you're using? Try increasing it a notch. If you're using 55gr, then maybe try the 62-63 gr. It sounds like you recently upgraded your buffer spring or buffer. If you replaced the buffer spring, be sure to try out your original buffer spring with it.
Also keep in mind that the upper may be a problem. Is the gas port closer or farther from the barrel? You might have to replace the buffer spring to some other length than what you currently have. If you currently have a carbine upper, then you need a carbine length buffer spring. |
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Quoted:
What is the load of the round you're using? Try increasing it a notch. If you're using 55gr, then maybe try the 62-63 gr. It sounds like you recently upgraded your buffer spring or buffer. If you replaced the buffer spring, be sure to try out your original buffer spring with it. Also keep in mind that the upper may be a problem. Is the gas port closer or farther from the barrel? You might have to replace the buffer spring to some other length than what you currently have. If you currently have a carbine upper, then you need a carbine length buffer spring. i was using pmc 55gr 5.56. i upgraded the buffer but not the spring but during shooting i put the old working buffer in and still had the same problem. The gas port was carbine length as was my old upper so that stayed the same. |
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Sounds like it's short stroking, did you thoroughly clean the chamber with a chamber brush and a good solvent? also did you lube the rifle good before shooting it? The brass marks are normal and I doubt the BCG is the culprit if it functioned fine in another rifle.
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| Well last night I lubed evertthing up and had a buddy help me out and took the upper out with him to the range with a magazine and some of the ammo I use. According to him it fired without a hiccup. So now that narrows it down to either the lack of lubrication when I shot or an issue with my lower. I guess I'll test it this week with my lower and hope that it was just the lube. If not I'm assuming its gotta be the buffer as thats the only thing I changed. |
| I shot again today and first shot jammed but then the next 3-4 mags went flawless. then occational jam again. this time though the jam is from the empty casing still being in the chamber. its not a lube problem as i just bought a niboron bcg and lubed that up. the one constant though was that the bolt doesn't lock back on the last shot which makes me think that maybe the bolt goes back just enough on most shots but has the occasional jam where it doesn't go back enough. that would explain why i can go through some mags without jamming. so without enough force bringing the bolt back what can it be? gas tube looks fine, gas block is tight. could it be a defective barrel in any way? weird thing is, is that my buddy said when he used my upper the bolt locked back on the last shot. could a buffer affect that in anyway? |
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not sure if this is normal but there seems to be a lot of carbon build up around the gas tube by the gas block and around the roll pin area...
http://photos.e46fanatics.com/data/500/medium/20130911_221414_resized.jpg http://photos.e46fanatics.com/data/500/medium/20130911_221403_resized.jpg http://photos.e46fanatics.com/data/500/medium/20130911_204322_resized.jpg |
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What is the last pic of exactly? I can't quite tell. And is than a mis-alignment? If your gas block is abstracting the gas port, you can get these symptoms, and if the gas block is bleeding off gas you can get these symptoms. that last pic is the roll pin holding the gas tube in the gas block. I just decided to test the gas flow and blew compressed air into the gas tube from the upper side. air definitely was coming out of the connection between the gas tube and gas block. is that normal or is there an issue with either the seal of the gas tube or alignment of the gas block? |
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What does your barrel's gas port diameter measure? Colt specs the USGI M4 gas port diameter at 0.0625" / 0.063". Also, what does your action/buffer spring measure? A carbine spring's free length (not compressed) should measure from a minimum length of 10-1/16" to a maximum length of 11-1/4".
My setup uses a 14.5" M4 barrel with a Colt spec'd carbine gas port diameter. I use Tactical Springs LLC (Sprinco USA) enhanced action/buffer (blue) spring, their ejector spring, their 5-coil extractor spring w/ black insert & viton o-ring, and a H2 buffer. I so far have shot American Eagle .223 REM 55gr. FMJ, Federal XM193, and Georiga Arms SS-109 62gr. w/ Lake City Brass Reloads with zero issues. I based my spring upgrades and buffer weight off of former JSOC member Mike Pannone's article, "The Big M4 Myth." By using a H3 buffer, enhanced action/buffer (blue) spring, and a 5-coil extractor spring w/ black insert & viton o-ring he demonstrated an AR-15 with USGI M4 gas port specs with a 14.5" or 16" barrel fully degreased and oil free can achieve a 2,400 round count before a weapon malfunction occurred. The Big M4 Myth http://www.defensereview.com/the-big-m4-myth-fouling-caused-by-the-direct-impingement-gas-system-makes-the-m4-unreliable/ |
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OP - stop changing multiple things before testing.
It sounds like it is short stroking and if you have gone through the gas system then it may be the buffer and or spring. As previously mentioned measure the length of the buffer spring uncompressed. What buffer and spring are you using? Can you describe the jams you are experiencing in more detail? A pic would be nice. |
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Quoted:
OP - stop changing multiple things before testing. It sounds like it is short stroking and if you have gone through the gas system then it may be the buffer and or spring. As previously mentioned measure the length of the buffer spring uncompressed. What buffer and spring are you using? Can you describe the jams you are experiencing in more detail? A pic would be nice. my spring is just a stock carbine spring from a mp15. buffer is spikes s2-t2. And in hindsight it did hurt me to change the whole upper at once. Basically the jams are for the most part double feeding where the empty casing would not fully eject and then the next round would jam. Also the bolt doesn't lock back after the last shot. I'm not so sure its not a gas issue anymore but if it is I don't know what it could be. I bought and changed the gas tube which results in same issue. I guess the only thing left is its either the gas block or the gas port diameter is too small. I brought it to my local gunsmith to try to figure out as i've about reached my limit on my DIY knowledge. Hopefully they can figure it out. keep everyone posted |
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