ar15 light primer strikes go away and then come back

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ldlfh7

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I have been scratching my head on this one. Went from a fixed A2 stock on my ar15 to a magpul adjustable. Everything was fine for sometime but then the occasional light primer strike starting happening. Did some research and realized I did not stake the castle nut, hence it was out of spec. Tightened everything back up and staked the castle nut in place. Everything is good for a couple hundred rounds then light primer strikes start happening again.
To be clear, this is not ammo specific. All ammo is showing this behavior. Also, it is not every shot. Maybe 2 or maybe 10 out of a 30 round mag fail to fire due to light primer strikes. Put that round back in the gun and it goes bang. Very random and confusing to me.
Funny thing is the castle nut is still staked and in the same position after staking the 1st time. What could be going on? I am considering going back to the A2 stock since it gave me zero problems but really do prefer the aesthetics and functionality of the adjustable stock. Any ideas what to look for? I am lost at this point.

Thanks
 
Obviously, this wouldn't have anything to do with the stock swap, but the first thing I would do is check to make sure the hammer spring is installed correctly. It could just be coincidence that the problem is showing itself now. A stock swap really shouldn't be able to mess with the firing pin strength.
 
Did you install a new recoil spring and buffer with the stock change?

Hand loads ? Primers not fully seated, bottomed in primer pocket. Classic example of primers not fully seated when they go off on the 2nd attempt.

Is the bolt fully going into battery? possible sizing issues? Or crud in the chamber needs to be cleaned out. OAL could be long.

A stronger recoil spring can also cause feeding problems if it's not driving the bolt fully back. Lacking the energy to fully load a round. Remove the upper and drop a round into the chamber and press with the finger. Then invert the barrel and see if it falls our freely. Do this on 30 rounds since you say it always happen within that count. This will eliminate any sizing issue but not primers being high. But you can check for those using a straight edge.
 
A buffer is not going to effect primer strikes. If the bolt wasn't going in to battery, yes, buffer and spring would be suspect. And if it's not going fully in to battery you'd get no primer strike. Though you might see what appears to be a light strike due to the floating firing pin.

What spring and buffer are you using? What trigger is in the rifle? If you tap the forward assist does the rifle fire?
 
Debris in firing pin channel? Brass from a pierced primer. Brass shavings. Its moves around with each firing.

Clean bolt of crud?
 
There are a few things that cause light primer strikes but the buffer tube and castle nut have nothing to do with it.
The most common cause is the hammer spring being installed backwards. This is very common with new builders. I have also seen a new factory gun with the hammer spring in backwards. Here are two photos to show how it should look.
Hammer spring correctly installed on the hammer.
B7429536-F734-4EEE-A2C8-288A201BA919.jpeg

How it should look on the lower.
D444CB18-6CBA-4179-91DD-C7B256197ECF.jpeg

The next thing that could be wrong is the firing pin could be out of spec. and be a little short. Or the bolt stim could be to long. Install a new firing pin.
Now as 243winxd said, your BCG may need cleaning. But I have found that an AR with run dirty as long as it has enough oil in it.
 
I will install a new firing pin, clean the crud out and ensure the hammer spring is installed correctly.
Thanks for the help. Sounds like if any of these are the issue than it's a painless fix.
 
Lots of stuff has been covered here, but:

Guys very often neglect some very common root causes when this issue of intermittent mis-fires rears its ugly head - all of which yield the same failure mode.

If the hammer fall is closing the bolt the last little bit of travel into battery, that impact with the carrier acts like a crumple zone. Hitting it again with the bolt falling fully into battery yields a proper fire.

Lots of reasons this can happen, but most often it’s either oversized ammo (headspace or bullet jam), dirty chamber (carbon ring as an option), sluggish bolt lock-up, or sluggish bolt carrier travel - these last two often being either a spec issue or dirty/improper lube.
 
Varminterror said:
If the hammer fall is closing the bolt the last little bit of travel into battery, that impact with the carrier acts like a crumple zone. Hitting it again with the bolt falling fully into battery yields a proper fire.

Lots of reasons this can happen, but most often it’s either oversized ammo (headspace or bullet jam), dirty chamber (carbon ring as an option), sluggish bolt lock-up, or sluggish bolt carrier travel - these last two often being either a spec issue or dirty/improper lube.
Click to expand...
Ya beat me to it - this has been overwhelmingly the cause of light strikes for me. The resolution is to determine why the hammer fall is completing the 'return to battery' cycle instead of the BCG/buffer/buffer spring's speed/momentum.
 
I'm not disputing, but just trying to learn here. If the bolt didn't fully close the first time, don't you have to open the bolt to recock the hammer? So, why would the bolt close fully the second time if there was an issue with spring pressure/bolt speed/friction in the action/binding in the chamber? I can see where an oversized round might be corrected the first time the bolt closed and might chamber on the second attempt, but I think OP said he's using no reloads, so any factory round should chamber okay.
 
Cemetery21 said:
So, why would the bolt close fully the second time if there was an issue with spring pressure/bolt speed/friction in the action/binding in the chamber?
Click to expand...

The bolt closed the first time based on the action of the gun, which might be running sluggish due to either gas issues, spring issues or simple dirt.

The bolt closes the second time due to manual operation, which tends to pull the bolt to it's full cycle and let it fly with more force. Kind of like closing a pistol slide by pushing the slide stop versus accidentally causing a malfunction by riding the slide home.
 
As yet, the OP hasn't confirmed that he tried any of the suggestions. One thing that has not been pointed out is that "intermittent light strikes" is a very rare thing. You actually have light strikes all the time. Sometimes your ammo goes off. That's the deal. Makes troubleshooting a little simpler. Most often, real intermittent strikes are limited to certain auto pistols that employ a firing pin blocking plunger. If the plunger is not moving completely out of the way, you can get truly intermittent strikes.

Assuming that the hammer spring is installed correctly, get a firing pin protrusion gauge for a couple bucks and confirm that you have the required f.p. protrusion. If not, its time for a firing pin. Remove the upper and confirm that the bolt carrier clicks easily and all the way into battery. It really should not be that hard.
Wireman
 
Cemetery21 said:
If the bolt didn't fully close the first time, don't you have to open the bolt to recock the hammer? So, why would the bolt close fully the second time if there was an issue with spring pressure/bolt speed/friction in the action/binding in the chamber?
Click to expand...

We’re talking about rounds that ALMOST ALMOST ALMOST fit, but don’t quite let the bolt fully close into battery. A bolt gun would have sufficient mechanical advantage to crush fit these - that’s why we always full length size for Semiautos. So once they have been forced to fit the first time, they’re far more prone to fit the second time, as the initial insertion did just a little sizing on the case.

Troubleshooting advice for pistols in an AR thread doesn’t make sense, and short firing pins are rarely the cause for intermittent failure to fires in AR’s. AR bolt carriers don’t “click” anywhere, and troubleshooting ammo fitment in that manner won’t provide any fruitful information. It’s not hard to troubleshoot an AR, but it can be very difficult if you follow bad advice for bad troubleshooting practices which don’t actually resolve anything.
 
Varminterror said:
It’s not hard to troubleshoot an AR, but it can be very difficult if you follow bad advice for bad troubleshooting practices which don’t actually resolve anything.
Click to expand...

OP, I hope this isn’t bad advice. Give you gun a good cleaning. If you have intermittent anything going on, first thing is a good cleaning. I would concentrate on the bolt carrier group and the locking lugs first, and the trigger group second.
I have “once” had a trigger pin brake in two. The gun still ran, it just felt weird. Hard to explain. It also wallowed out the pin hole in the receiver. Imagine me surprise when the pin came out in two pieces.
 
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