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Posted: 3/23/2016 3:10:32 PM EDT
Hi all, I'm having a conundrum with an ar pistol I put together and was seeking some input to figure what the heck is going wrong.  
Fired 100% unsuppressed

Ok, so the issue is every so often she won't fire.    Examining the "dud" round I will sometimes see a faint primer strike or none at all.  The round will chamber but no bang bang.  Some of these duds will extract out with the bolt when I yank the ch back, some I have to finger bang out, and a couple I had to bust out the brass rod and tap the stuck cartridge out.
I rechamber the duds and and some will go bang the second time around and others will not go off at all, no primer strikes at all.  The catch is, those rounds which absolutely will not fire will 100% go off in my ppq everytime and my buddy's g34 and g17 and check fine thru dillon 9mm gauge

So I am now starting to lean towards the chamber itself.  The barrel is friggin accurate when it does shoot, and don't get me wrong she does eat, but every so often for some reason she doesn't want to eat what I feed her, which is problematic.  Out of every 100rds  I would say 20 on average will not fire in this ar pistol

I am looking for some xp and feedback on what really needs to be looked at, I 've read some older posts but wanted to give my full detail of the issue.  I am at ends here and realize that something needs to be corrected and am ready to move forward thru all the proper channels, but just looking for the right direction and try to isolate the issue as best as can.

Thanks all in advance!!
Dave


Link Posted: 3/23/2016 3:39:41 PM EDT
[#1]
A few thoughts:
-Maybe the chamber was machined just a hair too deep, so the firing pin isn't as snug up the cartridge as it should be.
-could the firing pin be worn or too short?
-Maybe the Bolt is a little too long and is stopping the firing pin from fully penetrating.

My guess would be the first one if you're also having trouble extracting the round; the rim may be a little too deep for the extractor to grab.  Unfortunately, I'm not sure what you can do about that short of getting a new barrel.  Try swapping out BCGs first if you can, maybe the issue is in there?
Link Posted: 3/24/2016 1:09:09 AM EDT
[#2]
Link Posted: 3/24/2016 10:34:52 AM EDT
[#3]
Thanks for the input guys, I appreciate it.   All the parts are new, so perhaps worn parts may not be the issue.  I had a chance to check some numbers though.  It looks like like the case head protrusion is only about .008 longer than the bolt pocket, sounds really borderline, which explains maybe why some short cases are not cooking off if I am understanding things right.

Both of your thoughts confirmed my suspicions and hopefully have the issue isolated.  I'm probably not going to mess with this one and get in touch with the barrel maker.

One thing I did find interesting,.... at first I thought it may have been a spring/ buffer issue, but found out the issue would still arise regardless of which, but the reults were peculiar. Here's what happened...
Unsuppressed
8+ oz heavy buffer/sprinco red = most frequent failures
8+ oz heavy buffer/sprinco blue= frequent failures
H3 buffer/sprinco red + spacer= frequent failures
H3 buffer/sprinco red+ 2.75$ quarters = least frequent failures

Everything fed and cycled with all combos and lrbho with the exception of failure to fire, but preloading the spring with the quarters gave me the least fail to fire it seemed.

Thanks again guys!!  Will have to see what the barrel guys say.
Link Posted: 4/1/2016 1:31:09 PM EDT
[#4]
What trigger are you using?
Link Posted: 4/26/2016 12:31:22 PM EDT
[#5]
Timney 3lb sst..I know its been a bit, but I had some issues with the ramped bolt carrier I had addressed before all out junking the barrel.  The roll pin holding the carrier weight in place kept walking out..bad.  I sent it back and had it resolved,  now no issues to speak of.  I was dead certain the chamber was off, but now since I've had the bolt fixed/replaced I have gone 1000 rds with no problems. Maybe the roll pin was binding in the receiver and not allowing the action to go back into full battery or was placing the fp position slightly off center with the hammer causing light strikes.      I also replaced the bolt catch which was starting to bend against the ejector, I went against my judgment and opted for the cheapest bolt catch on this build, so lesson learned on that one.  She's running like a top now and I have to admit I freaking love shooting this thang, haven't had it long but have been shooting the hell out of it.  All seems good now.

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