AR Won't Cock

O

oneoldsap

New member
I assembled a frankengun for a friend , and it's showing me a new trick . Upon firing , it will eject and feed , but will not cock the hammer . I can operate it manually and it will cock fine . The lower is an Anderson , the lower parts kit is DPMS . The upper is a complete PSA (Palmetto State Armory) w/ 16" barrel . I have an Anderson lower parts kit ,that I'm thinking of installing . Just to see if that makes any difference . Normally I'd start chasing a short stroke issue , but it ejects and feeds just fine ! Any ideas anybody ? :confused:
 
Make sure the hammer spring isn't in backwards.

AR15-HammerSpringInstall.jpg
 
Not sure about that. I always figured the backward hammer spring showed up as light(or none) primer strikes.
Do you mean the hammer is following the bolt? I'd look at the disconnector function. And I'd do it right away w/o using a loaded magazine unless you like the runaway train feeling. I can tell you how to check this function but won't bother if you already know.
 
Also check to make sure the grip screw isn't interfering with the function, and if it is make sure you used the grip washer.
 
Interesting idea about the grip screw, since he may have been removing his shooting hand from the grip to work the operating handle. This might be why it cocks the hammer when manually cycled, but not when it is being fired.

It'll be interesting to hear about the resolution to this problem.

Bart Noir
 
"Disconnector worn or"
"the lower parts kit is DPMS "
I was assuming this meant the parts were new.



" missing spring "
that would indicate an error in assembly
 
Mobuck, I assume you know how to check if the disconnector is giving this problem. It's pretty simple. I don't intend any insult.
 
My latest build had the same issue. For whatever reason, switching hammers with another gun fixes the problem. Now both lowers work. One was a mil spec hammer and the other an "enhanced" hammer. For whatever reason the hammer was compatible with the cmmg lpk and not the masdefense lpk that the hammer came with. It would cock the hammer only if my finger was off the trigger. This was in a 9mm if that makes any difference. Although I never confirmed it, it seemed as if though my hammer pin holes were slightly out of spec, could very well be the issue with your Anderson. Do you have another lower to swap the hammer into and see of it works in that? And if it doesn't, then you could rule out and issue with the lower specs
 
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Yes, I do know how to check disconnector function, thanks.
Here's my policy: hammer following bolt is VERY bad. All of the parts involved are suspect and need to be immediately replaced or the defective part found and replaced.
This sort of thing will continue to show up with so many folks "building" AR's. Out-of-spec, incompatible, or worn parts get matched up and sooner or later, an accident is going to happen. Incorrect initial test fire procedures, lax assembly process, or just plain mistakes can end up with a run away or other unsafe condition. If that happens in public, it might result in an uncomfortable visit from the cops/ATF. Worse, it could end up as an injury/fatality.
I'm not pointing fingers at the OP(or anyone else) specifically but we owe it to ourselves and the shooting sports to make sure this doesn't happen.
 
I wonder, if the OP is holding the trigger back when testing the gun manually.

I've had problems with other types of semi-autos that I couldn't figure out due to me not holding the trigger back during function check.

A hammer following the bolt probably won't show up during a function check unless the trigger is held back as it is in real firing.
 
When cocking the hammer, by manually pulling the bolt back, the hammer cocks on the sear notch by the sear hook on the trigger. (Here, it is according to which trigger and hammer design you're using, as they have to match). After firing, the hammer catches the disconnector hook, using a different hammer notch, and when you release the trigger, the hammer is let loose by the disconnector, and is grabbed by the sear in the sear notch. If it is not cocking when firing, the very first thing to check is the disconnector.
 
Every lower I assemble is hand checked about 25 times for disconnector function including various trigger positions during cocking. The function is also tested during the first 5 shots of the test fire. Better safe than sorry.
 
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