Slam fires in my AR 308

arnie19

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Does anyone have any experience with AR slam fires and what can cause
them ?.I'm having this happen every so often with my 308 . I single feed it
and once in a great while it will slam fire when I release the bolt and let
it fly home using a Bob Sled . This happened twice to me Tuesday and once a
couple weeks ago . I later loaded 11 rounds I seated to mag length with Win.
cases and shot all 11 out of the mag with no problem . I have read that
maybe single feeding with out letting the bolt strip the cartridge out of
the mag causes too much momentum and causes the firing pin to fire from
inertia . Mines a DPMS style and I understand other types use a spring to
keep the firing pin from doing this . It could be a combination of this and
using Federal 210m primers . The cases were IVI which is Canadian version of
our Lake City . Any ideas or maybe I just have to hold the bolt back from
flying so hard home . Arnie
 

Scottmilk9

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Take your bolt apart and make sure to clean the channel where the firing pin is. Sometimes debris can keep the firing pin stuck in the forward position. An AR bolt slamming home should not set off a primer, the firing pin is free floating. I don't know much about the ammo you are using, but have a hard time believing that the primers are that sensitive to the momentum of the bolt carrier going forward.
Another thing to check is your trigger, make sure the sear engagement is fine, thats another way it can go off.
 

arnie19

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inalabama,did you have slam fires before seating primers deeper ?? I guess I will use my primer pocket uniformer and cut all the pockets as deep as it will go and see if that helps . I will also take the carrier apart and clean it up .It was clean but I lubed the firing pin and the bolt with frog lube and maybe its too thick and doesn't allow the firing pin to slide easy . I have 4 other AR's and don't have this problem with any of them and my friend single loads his 308 the same way with out my problem . He is using Win. brass so maybe his primers are seating deeper .I will have to check that out next week .
 

Ballista One

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Another thing to check is your trigger, make sure the sear engagement is fine, thats another way it can go off.

This...

The most likely suspect in what many shooters refer to as a "slam fire" is a deficiency in the fire control group. Especially with adjustable triggers. That being said, the mechanical safety in the bolt group is a very robust design and I'd argue that slam fire induced by the firing pin is improbable.
 

KYbulldog

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FCG? As in too strong a hammer spring? I wonder why Armalite designed the spring into their BCG then? I have tested several different brands and projectile weights with hand feeding(not letting the gun strip from mag) and have not had one slam fire yet. Spring works good I'd say.

AR stands for Armalite. (yeah, I'm biased)
 

samnev

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    I have found that the Federal primers are the ones most often associated with slam fires when I have had it happen to me or seen it happen to others at the range. Although I have seen it happen with other brands as well. I am assuming you have seated your primers to the correct depth and your coal is correct if that is the case I would think the next cause is primer sensitivity. I have had slam fires in the past and each time it was Federal primers. Now I primarily use CCI #34 mil spec primers in my 308/7.62 semi autos and CCI #41 mil spec primers in my 556/223 semi autos. On occasion when I have run out of either of those I have used Winchester primers and have not had a slam fire since then.
     

    arnie19

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    I took 200 of those IVI cases and cut the primer pockets with a Sinclair tool .There were some that were pretty shallow . I also did a little experiment and primed (with federal 210 m )some cases that were as made new .I managed to get two of them to slam fire after a few tries . I then pulled the trigger and the hammer fell so it isn't the hammer causing it .Im sure its the momentum of the firing pin . I guess I will just not single feed this rifle with any primer other than CCI #34 Military primers and until my present loads are gone ,I will ease the bolt closed rather than let it slam. I have a small mill and lathe .I will look into trying to put a spring on the firing pin .I wonder if an Armalite carrier and firing pin would work in a DPMS style upper ? I bet the head would not work because of head space or might not work on the extension.
     

    usmc02xx

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    FCG? As in too strong a hammer spring? I wonder why Armalite designed the spring into their BCG then? I have tested several different brands and projectile weights with hand feeding(not letting the gun strip from mag) and have not had one slam fire yet. Spring works good I'd say.

    AR stands for Armalite. (yeah, I'm biased)

    i think hes referring to people adjusting triggers too low and it losing sear engagment with the shock of bolt going forward. Heavy 308 bolts make a lot of momentum. Hell, I had mine set at 2 lbs and would accidentally double tap every now and then from the recoil lol. I took it back up to 3 lbs and it never did it again. Or should i say i never did it again.
     

    Oleshep

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    Are you seating the primers fully?

    a high primer could do it. i place the cases in an ammo box after priming and use a mag glass to examine the primers. If they are seated properly you can see a lip of case above the primer
     

    Ballista One

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    I think he's referring to people adjusting triggers too low and it losing sear engagement with the shock of bolt going forward.

    Exactly...

    I'm sure its the momentum of the firing pin.

    You really can't be sure until you have exhausted all of the other possibilities. The primary purpose of the spring on the firing pin is to prevent an AD while cocked and locked. You will find that adding a spring to an AR15 is problematic for a number of reasons.

    I'd argue that if all of the parts that make up a bolt group are serviceable and dimensionally correct, it's impossible for a "slam fire" to occur.
     
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    mtrmn

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    EVERY round I chamber by hand or using the bolt release will have a slight dimple where the firing pin hit the primer due to momentum when the bolt slammed shut. I always use the CCI military primers exclusively nowadays just to ease my mind, but back when I never knew they existed I used the regular CCI primers and I cannot remember ever having a slamfire. I have read here and elsewhere that Federal primers are the softest ones out there and people have claimed they cured slamfire problems by getting away from the Federal primers.

    IF the hammer actually fell when the bolt slammed home, you would not be able to prove that it didn't by simply pulling the trigger again--the bolt carrier cocks the hammer when the gun cycles. Therefore when you had your "slamfire" the hammer should have reset when the gun ejected. Unless you have the gas shut off or something and I missed that detail in your post.

    ETA: To detect high primers all you have to do is run your finger across each loaded round. The primer surface should be slightly lower than the head of the case and this is easily detected by feel. Bear in mind that I AM NOT A SNIPER, BENCHRESTER OR COMPETITOR so I scoff at buying a depth gage that I could easily substitute my dial caliper for. Just use the depth indicator on the rear end of the caliper.
     
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    BravoSierra

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    If its FCG, may want to read up on the Hiperfire trigger. The design, supposedly helps remedy this, while keeping the pull weight low. Very interesting design..
     

    sic65stang

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    Use the mag and this wont happen. Single loading can cause this cause the free floating pin. I have experienced it.

    It can be that there is a high primer, but I would bet on it being because the single loading practice cause it seems to happen so often. JMO
     

    judgedelta

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    I have read of people putting a half of a mag release spring behind the firing pin to keep it from slamming forward. Haven't tried it myself, but you should be able to find it on google or search SH.
     

    samnev

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    Quote from mtrmn: "ETA: To detect high primers all you have to do is run your finger across each loaded round. The primer surface should be slightly lower than the head of the case and this is easily detected by feel. Bear in mind that I AM NOT A SNIPER, BENCHRESTER OR COMPETITOR so I scoff at buying a depth gage that I could easily substitute my dial caliper for. Just use the depth indicator on the rear end of the caliper."
    +1