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Posted: 10/20/2015 3:12:44 AM EDT
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This is my 4th build and finally, ran into a problem.
I'm getting light primer strikes 50% of the time. The other 50% of the time, it shoots and cycles fine and even locks back on an empty mag. When it doesn't fire, I pull the charging handle back and the round is still sitting in the chamber. I Don't think the extractor is hooking onto the brass case to eject it. I can easily remove the unfired round with a rod. When I attempt to cycle rounds manually, some rounds don't hook and eject. I am loading one round in the mag every time and I make sure the bolt closes all the way every time. I'v already tried a different BCG, ammo, and magazines and these work fine in my other AR. I'm using an expensive KAK pistol barrel, if that matters. |
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Hook a spent case on the extractor, and cam the ejector in a few times to make sure that the ejector is not binding in it spring channel (will be under a lot of spring tension, but should not be binding.
Next, check the extractor to see if it has a O ring installed around the extractor spring. If it has an O ring, remove the O ring since it may be putting too much tension on the extractor and not able to correctly snap over the rim of the round at feeding. Also, would be a good time to clean the extractor and its bolt channel as well. Lastly, post a photo of your bolt face with extractor installed. Hence the extractor could be out of spec, with it too far towards the inside of the bolt face, and the case rim is unable to push the extractor outwards at loading for the extractor to get over the case rim. Good relationship of the extractor round rim entry angle to the recess in the bolt face, 
Extractor out of spec, and it case rim entry angle is too far inward on the bolt instead, 
Bluntly put, the bolt will feed the round out of the mag and into the chamber, and as the bolt slams home on the round and it about to cam over to lock up with barrel lugs inside the barrel extension, that is when the extractor slips over the rim to lock it in place. Also to point out, if the extractor does not slip over the rim, does not lock up in the bolt, the hammer striking the firing pin will not ignite the primer (FP is blocked from reaching full protrusion). To see the firing pin block, pull the B/C and look at the back of the carrier when the firing pin enters it. Hence until the bolt is locked home, the tail of the bolt will not protrude out the back of the carrier to allow the FP to reach full protrusion out the bolt face. So again, your problem with the failure to fire is that the extractor is not slipping over the case rim at loading, which is preventing the bolt from locking home, which in turn is preventing the protrusion of the FP out of the bolt face. Since the FP block is working correctly, we now focus on what is preventing the extractor from slipping over the rim at loading instead. Hence either too much extractor spring tension, or the entry rim ramp on the extractor too far forward, and instead of the case rim hitting that ramp to wedge the extractor open, it hitting the outer flat of the extractor instead. |
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