AR primer strike on feed

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  • BigCountry14

    Ultimate Member
    MDS Supporter
    Jan 17, 2013
    1,668
    I've been working to remove a coyote on my property for the last few weeks. I've got a suppressed .300 blackout AR that I'm hunting with. Tonight I finally got a shot on one of the critters. When I came into the house and unloaded everything, I noticed that the round that was chambered after I had fired had a light primer strike on it.

    Now I'm a little concerned. The rifle itself has had several hundred rounds through it, I've never had an issue and never have seen this before (although I usually would shoot a mag, not a single shot, so who knows). Shot earlier in the day did not produce the same result.

    I understand that slam fires on AR are fairly rare, and google wasn't much help. A lot of results for light primer strikes, but more so in the failure to fire realm than on feed.

    The lower was is a Stag, and it was complete when I got it. The upper is an AAC 16 inch barrel. Im not sure where to start. Any suggestions on what to look at?

    Sent from my SM-G960U using Tapatalk
     

    Mule

    Just Mule
    MDS Supporter
    Sep 14, 2013
    652
    I've encountered that before, with properly gauged rifles, both factory and homebuilt.

    My understanding is that what you're describing may not be unexpected, depending on the depth of the dimple, and is related to the free-floating firing pin.

    Here's an explanation, and pics, from an industry source:

    https://www.windhamweaponry.com/dimpled-primers-after-chambering/

    I'd suggest checking how far your firing pin protrudes from the bolt face, with a firing pin protrusion gauge. If you don't have one of those, a caliper with a depth gauge should do the trick. Max spec is .036". If your current firing pin extends farther than that, I'd replace it with another firing pin that's in spec.

    With that being said, don't rely on me. Have it looked at by a professional.
    :)
     
    Last edited:

    John from MD

    American Patriot
    MDS Supporter
    May 12, 2005
    22,731
    Socialist State of Maryland
    What you experienced is normal for the AR platform as the firing pin has no spring and the bolt carrier's momentum will carry it forward. The weight of the firing pin keeps it from setting off the primer. Many people spend lots of extra money on #41 primers when there is no need.

    However, if you own a MAS 49, I strongly suggest you invest in #34 primers if you reload. You can also have your firing pin turned down on a lathe (I forget the exact size) and it will function with non mil spec primers with out slam firing.
     

    BigCountry14

    Ultimate Member
    MDS Supporter
    Jan 17, 2013
    1,668
    Thanks all. With all of the rounds I have run through my ARs, I guess I've never really stopped mid mag and never noticed.

    Sent from my SM-G960U using Tapatalk
     

    outrider58

    Eats Bacon Raw
    MDS Supporter
    Jul 29, 2014
    49,815
    What you experienced is normal for the AR platform as the firing pin has no spring and the bolt carrier's momentum will carry it forward. The weight of the firing pin keeps it from setting off the primer. Many people spend lots of extra money on #41 primers when there is no need.
    ^^^This^^^
    I load most of my .223/5.56 rounds with the very soft WSR primers. Never had a slam fire.
     

    JohnC

    Active Member
    May 29, 2019
    311
    Baltimore, MD
    As others have stated, normal and expected. You'll find the same dimple with many US rifles with free floating firing pins (AR platform, M1 Garand, M1a, M1 carbine). Hence the harder primers and making sure you seat the primers to the correct depth when re loading.
     

    Pinecone

    Ultimate Member
    MDS Supporter
    Feb 4, 2013
    28,175
    At least one person in MDS has had an AD from this.

    CCI recommends NOT loading .223 with #400 standard primers for ARs. Remington only recommends the 7 1/2 primer, not the standard.

    The CCI #450 SR Mag primer is very close to the #41 military primer. Same cup, same priming compound and same amount of priming compound. The #41 has a bit more space between the priming pellet and the anvil to help avoid slam fires.

    Do what you are comfortable with, but I only load #450 or Rem 7 1/2 primers for .223.
     

    Doco Overboard

    Ultimate Member
    I'm not aware of any manufacturers that approve of reloaded ammunition in any of their products.
    Ammunition selection is critical in any firearms application especially in any semi or fully automatic design even when newly manufactured. Uniforming primer pockets, and making sure cartridge headspace is important in avoiding slam or out of battery firing when floating firing pins are involved. Also, not over-seating primers or creating overly sensitized primers by crushing them is important.
    Making sure firing pins are not over oiled, greased or fouled excessively is another important factor.
    Firing pins should be inspected to be sure they are in spec, not stretched and have a hemispherical shape that does not allow excessive clearnaces at the firing pin port in the bolt face.
     

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