AR15 Double Fire/ Slam Firing


General Disarray
November 7, 2012, 12:41 AM
I have an Olympic lower that I don't know its origins. I bought this Franken build a while ago, and it ran ok so far. The searches I've done on slam fires tell of the same test for releasing the hammer, holding the trigger and resetting the hammer, then letting trigger go. I've done that test and when I let the trigger go, the nose catches the hammer as it should. I just cleaned the bolt/firing pin completely clean with Gun Scrubber aersol and reoiled, and it still slam fired on the 4th round today; then over and over every few rounds. What else besides the trigger, bolt face fouling, or high primers can cause this?

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November 7, 2012, 07:50 AM
What ammo are you shooting? Reloads, cheap surplus?

How worn is the bolt?

If the bolt looks ok, i would start with a trigger group. A trigger group is cheap and easy to replace.

November 7, 2012, 08:18 AM
I've always wondered about this. What are the main reasons that causes ARs to slam fire? Will they only slam-fire with a full-auto BCG?

November 7, 2012, 08:29 AM
The only difference between a full and semi BCG is a smaller cut out. Functionally they are the same. There is more mass, but that by it self shouldn't cause a slam fire. I have full auto BCGs in 2 of my ARs and have never had a slam fire.

November 7, 2012, 09:07 AM
debris under the trigger 'shoe'?

Shawn Dodson
November 7, 2012, 09:22 AM
An intermittent problem can be a challenge to diagnose.

I suggest you load a magazine with a single cartridge and seat it in your rifle. Fire several rounds, one at a time, using the magazine loaded with a single cartridge. Press and hold the trigger to the rear to fire each shot, then slowly release pressure on the trigger. The trigger will either reset or the hammer will drop completely. If you think the hammer dropped then press the trigger again. It will feel "dead". You can confirm that the hammer is uncocked by attempting to engage the manual safety. If you can't engage the manual safety then the hammer is indeed uncocked. You'll have to replace the hammer, the trigger or both.

It sounds like a previous owner did a "trigger job" by honing the hammer notch or trigger sear (or both) and removed to much material in the process or the parts have since worn because the case hardened surface was removed.

November 7, 2012, 09:30 AM
It sounds like someone did a little 'trigger work' on it. I butchered an AR trigger group one time and after a few months, the engagements wore a little and started giving doubles. Replaced the trigger, hammer (this is the part I butchered) and dis-connecter and the problem was solved.

November 7, 2012, 09:38 AM
Looks like you just bought an LPK.

Better get it before the panic-buying reaches full swell.

General Disarray
November 7, 2012, 03:51 PM
Looks like you just bought an LPK What's an LPK?

I didn't know much about the triggers on these, so watched a few videos etc, and took mine apart. Photos below. All the edges look solid, and springs are strong. Disconnector looks good, as does it's spring (not sure how long it should be though). I did the test for holding trigger, reset hammer, and let it go: the nose caught the hammer as it should. Does anyone see anything weird in the photos (I know the 2nd pin is not in photo):

November 7, 2012, 03:59 PM
I'm going to venture a wild guess here: mismatched parts. If the trigger group parts are from different manufacturers or batches, they can have different tolerances and not quite match up the way they should. The trigger seems to have a darker finish than the other two, which would support this.

November 7, 2012, 04:14 PM
Your triggers parts might be out of spec. Check your pin holes and make sure they are tight. Also check the firing pin and see if it free floats or if it's too long.
I haven't had a slam fire ever with any AR15. I have experienced it with an AK and SKS. One case was a sticky firing pin, another was soft primers. Both correctable and my own fault.

November 7, 2012, 04:46 PM
I would look at that disconnect first

General Disarray
November 7, 2012, 05:12 PM
I would look at that disconnect first

What would I be looking for there?

General Disarray
November 7, 2012, 05:27 PM
I'm comparing the disconnector with the one inside an unfired lower I have, and that one has sharp square edges. This one in suspect is in fact all worn along the front edge that catches the nose, and even more heavily along one side versus the other. It wasn't obvious at first, but adelbridge's suspicion was correct, the front edge of the disconnector is worn all uneven. It's still catching the nose when I do that hold the trigger/set the hammer test, so I figured it must be good to go. Maybe its worn just enough to function sometime, but not always under firing conditions?

November 7, 2012, 05:50 PM
The disconnector and/or disco spring may just be worn out. The catch on the hammer where the disconnector latches looks a bit worn in the pics also, but it is kind of hard to tell with the pics.

There is a lot more force and vibration under fire conditions then when hand cycling the bolt.

You pulled the firing pin out and verified that the channel was clear also? Sometimes you hear about small pieces of popped primers getting caught b/w the pin and bolt and causing slamfires. I would not consider a worn disconnector causing hammer follow to be a slamfire, but that is kind of nit picking I guess.

November 7, 2012, 06:14 PM
1- Replace the all the internals with a quality lpk. This is a relatively inexpensive fix.
2. Make sure your free floating firing pin is indeed free floating and not sluggish with accumulated crud.

November 7, 2012, 06:24 PM
Reloads with soft primers may do that from what I understand. Federals, and some Winchesters have that reputation.

I haven't seen it yet, but I hear of it from time to time.

CCI makes primers specifically for ARs, the CCI #41.

But you don't say if you're shooting factory or handloads.

November 7, 2012, 06:40 PM
Different deal, but my friend had a slam fire with his AR. Reloads with Federal (soft) primers being single loaded. The bolt was released from locked position. Forward momentum without picking up a round to slow down the bolt caused the firing pin to hit the primer hard enough to ignite.

I realize it's a different deal, but something to watch.

November 8, 2012, 05:22 AM
Better get it before the panic-buying reaches full swell.
Way to start the panic buying!

November 25, 2012, 12:43 AM
I just joined this forum to sell a Jewell ar trigger, still trying to figure out how to post an ad when I saw this post.
I have an essentially new (less than 50 shots) jewell ar-15 2 stage trigger.
I think it was @ $215 when I bought from Jewell.
I have all original tools and intructions and a spare hammer spring.
This is for an ar-15 and I have an ar-10,
it worked in the 10, but is not made for it, so I'm getting a different trigger.
It works perfectly, and is adjustable in both stages with a small allen wrench.
I'm going to ask $150 when I figure out how to post,
it would probably cure the slam fires and is very smooth and crisp,
reportedly the best match trigger made for the ar-15,
let me know if your interested.

November 25, 2012, 01:09 AM
I'd also buy a LPK and then replace your trigger group. Odds are very high that will solve your problem. It's nice to have the extra pins, springs, etc. from the LPK just in case. Simple, cheap fix.

November 25, 2012, 10:52 AM
Slamfires from overly sensitive primers are a rare event. Rare enough that the Army took the early AR15ís through development and field testing before enough slamfires occurred that it got high level attention. Your rifle should not be slamfiring every fourth round based on the statistics of primer sensitivity alone. But, funny things happen.

Constant slamfires are typically mechanical in nature, double check that your trigger mechanism is not following, and make sure your primers are below the case head.

If that is not it, use CCI #41ís or Tula Milspec primers.

It is a fact of historical record that AR15ís did slamfire, and incidentally still do. The Army lightened the firing pin and used a less sensitive primer.

Primer Sensitivity

Initial Specifications. Ammunition specifications established by the Air Force on 24 January 1963 provided for quality control against cocked, inverted, loose, and nicked primers. The specifications further provided for inspection and test of waterproofing
and the crimp of primers. However, the specifications did not provide for specific limitations on primer sensitivity for 5.56mm ammunition.-

Development. At the first meeting of the Technical Coordinating Committee on 26 March 1963,1 the Air Force representatives submitted a list of reported ammunition deficiencies, which included "high primers" and "primers too sensitive". It was agreed that Frankford Arsenal would investigate the matter and recommend corrective action.

One of the malfunctions reported by the Air Force was the premature firing of cartridges that occurred upon initial charging of the M16 rifle with a cartridge from the magazine, or upon singleloading of a cartridge directly into the chamber, or when two rounds were fired at one trigger pull during semiautomatic fire.

This malfunction was attributed to "high" or protruding primers, although the tests did not confirm this theory.

However, analysis indicates that if high primers caused the premature firing, the firing should have occurred upon impact of the bolt face with the protruding primer. At this point in the weapon cycle, the bolt head would not. have been rotated to the locked position by action of the cam pin and carrier. Had firing occurred with the bolt in the unlocked position, it would have resulted in a blow back and would not have been undetected. No such disruptions were reported-. Since premature firing occurred after bolt-locking, it must have coincided in time with the impact of the bolt carrier against the bolt head. At the instant of impact, the "free floating" firing pin is moving at the velocity of the bolt carrier. The kinetic energy of the pin must be dissipated by such frictional forces as it encounters in the forward movement, and, finally, in impact of the firing pin tip with the primer of the chambered cartridge. This premise was confirmed by the visible indentation appearing on cartridges which were chambered by the mechanism and extracted unfired.

Frankford Arsenal identified test procedures for measuring firing pin energy and recommended limits for primer sensitivity.

Early heavy firing pin on top. Picture from

November 25, 2012, 11:24 AM
1) High primers if using reloads.
2) That hammer sear looks more polished than most I see out of the box.

If you're not shooting reloads change the trigger, hammer, and disconnector.

Also, make sure you're not bump firing the rifle w/o realizing it.


November 25, 2012, 12:06 PM
My RRA two stage trigger doubled on me twice at the range one day. I went home, took the parts out, threw them away, and ordered another trigger group. No problems since. I bought a Spikes ( It is nothing special as far as triggers go, but it is a reasonable pull weight, reasonably smooth, and most importantly, it has been 100% dependable. If I need to buy another trigger group, I think I am going to try this one (

Of course the best option out there is the Geisselle.

November 26, 2012, 10:11 PM
Scanned through the answers quickly so this may have been mentioned. Check the firing pin hole in the bolt face for the possibility of the firing pin getting stuck. The hole may look alright but be a tad enlarged. Also maybe try another pin... they're cheap.

A thing I've had happen... using a two stage trigger I started to what I call diddle the trigger, that is only release it enough to regain the second stage. Holding the trigger right at the point where it gets released by the disconnector can cause double fires because the jarring of the rifle is enough to cause the sear to release. But I suppose you would know if you were doing that.

November 26, 2012, 10:19 PM
An AR-15 firing pin cannot get stuck in the foreword position unless it is broke in two.

When the bolt carrier is blown back off the bolt to unlock it, the firing pin is mechanically retracted and is shorter then the bolt body.

The only time it is able to reach a primer is when the bolt is telescoped back inside the carrier and fully locked in the barrel extension.

Doubling can be caused by only one of three things:
1. Defective trigger / disconnecter.
2. Soft or too sensitive primers getting set off just as the bolt is fully locked shut and the firing pin is able to fly foreword and hit it.
3. A very light trigger, combined with a relaxed hold on the gun, allowing the gun to "bump fire" against your trigger finger.


November 26, 2012, 11:11 PM
I had an AR go full auto when one of the FCG pins backed out (can't remember which one for sure, but I think it was the trigger). Didn't dump the whole mag, just kept firing as long as the trigger was pulled back. Interestingly, it did it with a 22lr adapter and with the 5.56 BCG. That problem would seem easy to see, but someone else had to show it to me because I was so flustered by the problem.

November 28, 2012, 12:03 AM
My BIL has an older AR that will double fire when he, and he alone shoots it. 3 other shooters (myself included) could not induce his issue trying every trigger handling trick in the book. He would shoot it, double fire. We would shoot it, 1 pull, 1 fire. I even tried the famous artillery hold we used to shoot powerful springer air rifles, with no success duplicating his issue.

Obviously a bump-fire issue, with his trigger or hold technique, but he has tried every trigger drill out there, without success.

He's going to replace the trigger-group, and go from there. I'm suspecting worn components and holding too loosely, but it should only fire once, no matter how held.

Very odd, that 3 of us couldn't duplicate his issue, and we all fired it many times.

The rifle is defective, that much we all know.

November 29, 2012, 05:05 PM
Wow. Such a magnificent rifle, the Armalite.

I am so Glad for the spring loaded firing pin and the FCG on my SIG556 and my PTR91. No issues like this at all.


November 30, 2012, 10:35 AM
Reloads with soft primers may do that from what I understand. Federals, and some Winchesters have that reputation.
If youre using reloads, especially with Winchester primers, I wouldnt get to concerned. Just switch out the Winchesters for CCI.

I got a couple of thousand Winchester small rifle primers from a buddy, and figured Id burn them up, so I loaded up a bunch for my one Armalite. I usually use CCI, and never had any issues with slamfires with my reloads using them. With the Winchesters, from the very first mag, I was getting doubles and triples with just about every pull of the trigger. Switch out the mag for one with CCI's, and the problem immediately stopped.

November 30, 2012, 10:54 AM
I've seen brand new SIG M400 AR15s doubling because of the trigger pins walking out. I'm not sure what happens when the pins start walking out but could be something similar.

November 30, 2012, 09:55 PM
I've seen brand new SIG M400 AR15s doubling because of the trigger pins walking out. I'm not sure what happens when the pins start walking out but could be something similar.
That's why I always use KNS Anti-Rotation pins in all my ARs.

Ounce of prevention...

December 1, 2012, 07:26 AM
Wow. Such a magnificent rifle, the Armalite.

I am so Glad for the spring loaded firing pin and the FCG on my SIG556 and my PTR91. No issues like this at all.

Firing pin spring is standard on the ArmaLite AR-10. It is not needed on the AR15.

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