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Posted: 11/23/2007 1:50:07 PM EDT
I have a factory built colt 9mm AR not a conversion which seems to slamfire every other round or so in 2-3 round bursts, I've detail stripped it, cleaned it rather thoroughly and inspected the gun within the bounds of my knowledge and can't seem to find anything visibly wrong. I've also tried quite a few different brands of ammo.

I've been tossing a few things back and forth in my head which might be the cause.

Headspacing, maybe the headspace is so dramatically off it's causing it to slamfire?

The other thing I was considering is the buffer spring, now I can't reasonably find a reason for a weak spring to cause something to slamfire but maybe if the blowback operation is starting too early and happening too quickly there is not enough chamber pressure to slam the bolt back far enough to engage the catch?

Any input would be more than welcome, I've spoken to a couple gunsmiths but they're obligated by law to dispose of an "unsafe" weapon in the event that they cannot fix it. So I'm concerned with handing it over to someone and fining out it's gone, would much rather try to troubleshoot it myself but there isn't a whole lot of information on 9mm AR-15 issues.
Link Posted: 11/23/2007 6:18:16 PM EDT
Link Posted: 11/23/2007 9:27:00 PM EDT
[Last Edit: 11/23/2007 9:30:46 PM EDT by Dano523]
Link Posted: 11/24/2007 2:56:39 AM EDT
[Last Edit: 11/24/2007 5:18:36 AM EDT by Reoze]
I'll definitely try the steps above, as far as the gunsmiths I'm attempting to remember who they were, it was a few years ago, this gun has been locked up ever since.

Also thanks for the welcome.

9mm's don't "headspace" like a rotating bolt, it works just like a pistol. The bolt does not "lock" meaning there is no "timing" so when the bullet heads down the barrel, the bolt heads rearward. Your issue will most likely be firing pin binding or FCG.

Whoops, Thanks for the heads up though, I remember getting rather anal retentive about my M1A headspace as it was a "service" rifle and was known to slamfire if my reloads headspacing was off.
Link Posted: 11/24/2007 3:03:00 AM EDT

Originally Posted By PropDoc:

Check for a broken hammer or trigger pin.

Yep, 9mm set-ups are notoriously hard on the pins.
Link Posted: 11/24/2007 6:41:18 AM EDT
* The old style colts took .171 pins and had the same FCG hole sizes. The new colts take .154 pins/FCG parts and if you have .171 parts on .154 pins, you are going to have problems from the slop (read pins and FCG parts need to be the correct size for the holes in the receiver).

This is exactly the case, Thanks for the help
Link Posted: 11/28/2007 11:28:50 AM EDT
Just got in a new RRA hammer and it works great, just wanted to say thanks for the help, The hammer pin width would have been the last thing to cross my mind as an issue.
Link Posted: 12/2/2007 3:20:13 PM EDT
Maybe you should post the names of those "gunsmiths" who sound like they need a visit from the nutcracker.
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