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Posted: 3/8/2007 4:49:51 AM EDT
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I am working on a Colt m16A1 with a collapsible stock for a coworker. He wanted 20" barrel replaced with a more "entry friendly" barrel. I mounted a DPMS 16" heavy bbl with a 4 rail gas block for him. I'm now finding that the rifle functions 100% flawlessly in semi-auto, but when I switch to full auto the rifle will fire an indiscriminate number of rounds then stops, chambering the next round, which will have a firing pin indention on the primer. The number of rounds fired in the groups before failure tend to descend numerically by 1 round until it is firing one round and stopping. I spoke with someone from the company I did my armorer's school at. He advised that I was experiencing bolt bounce and the problem could be corrected with the use of a heavy buffer or hydraulic (enidine) buffer. I installed the Enidine buffer. It seems to improve the number of rounds fired, but doesn't cure the problem. At some point the next round will still fail to fire and the cycle I just described will again repeat itself until I am once again firing 1 round then failure to fire on the subsequently loaded cartridge. Any help would be greatly appreciated, as I need to, if at all possible have it operational by this coming monday. Thank you in advance |
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If you are using the old upper receiver and old B/C, then the sear release timing should have not changed, But check it anyway just in case. If you find the carrier gap (front of the carrier to the face of the receiver extension at hammer release off the auto sear) is larger than .090, you may need to retime the sear or replace parts (should be in the .083 range). Next item up would be the buffer spring, If the spring is in question (service life), change it out with a new unit. With a worn spring, the buffer gets sling shot'd off the back of the receiver extension (bolt unlock speed), and will increase bolt bounce. Simply, if the rifle will run in semi, then it's timing or bounce issue at hand. |
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No worrier, and to check timing, just use a few drill bits to wedge in between the carrier face and barrel extension to see what the gap of the two are when the sear drops the hammer (remember to hold the trigger back, the selector in full as you walk the charging handle back forward). I have a small stepped rod (.078 on one side and.090 on the other, and yes it's nice to have a lathe to make your own test tools, but you can use the back side of drill bits in the mean time) that I use to field test timing. The sear should release the hammer with the .078 installed, but not with the .090 side holding the carrier off the barrel extension face. |
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