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Thread: Let's Talk about Light Primer Strikes...

  1. #1
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    Let's Talk about Light Primer Strikes...

    Ok guys,
    Got my first taste of the light primer strikes actually causing a FTF today.
    Granted, for years I've known about them, and I always keep an eye on the first round in my mag, which gets chambered numerous times over and over.
    However, this was the first time that I've ever actually had a round NOT go off. - FTF. It was a quality round too. (75gr TAP FPD).
    SO my questions are as follows:

    1. Will a GAS PISTON rifle ALSO cause light firing pin/primer strikes, when chambering/de-chambering rounds repeatedly??? (hell, I say "repeatedly", but my DI Colt causes a light FP strike on the primer after only ONE SINGLE chambering/dechambering of a round. I was just wondering if a gas piston rifle somehow differs from a DI gun with regards to light primer strikes????

    2. What do you Mil/LEO guys do when your lives depend on your rifles? Do you keep track of how many strikes a round has taken, and then rotate it out of the lineup, OR do you toss the round, or what???


    Can someone explain to me just WHY this happens? Maybe it's late, and I'm tired, but I'm not really grasping what causes a light primer strike?
    Is this something that also happens with AR10's, or how about AK's??? ( I assume it's not an AK thing, but thought I'd ask)...
    Thanks...

  2. #2
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    Light primer strikes (not the marking of the primer from chambering) can be caused by:

    1) blunted firing pin or firing pin that falls below the minimum measurement on a firing pin protrusion gauge
    2) REALLY excessive headspace (another clue of excessive headspace would be wandering groups)
    3) hard primer
    4) deep seated primer
    5) weak or broken hammer spring
    6) improperly assembled hammer spring

    I've only had one gun with light primer strikes, and it was a M1 carbine with excessive headspace. I've shot a good amount of rounds through my AK clone, AR10, and AR15s, and have not experienced light primer strike.
    Nemo me impune lacessit

  3. #3
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    Ok, then let me correct myself.
    I am NOT talking about light primer strikes. I am INDEED inquiring about the striking of the primer during chambering/dechambering...
    Please excuse the incorrect terminology...

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    Big difference Paul...

    The free floating firing pin will leave an impression on the firing pin, and that is no big deal.
    However, what you are doing is breaking the seal between the bullet and the case. Done repeatedly, it is enough to cause problems.
    I was working with FAST Co in Bahrain pre 911, and we have over 100 Type 2 Malfunctions with the M9 (something beretta says is impossible) in one iteration.. We figured out that this was guard ammo that they were using up. After repeated chamberings / unloadings the seal was broken sufficiently and the humidity high enough/ long enough where powder contamination was possible.
    My SOP is 1 cycle. After that, it is practice ammo.

    FYI, while Hornady is a quality maker, they are not without problems. We have see enough failures to fire on that 75 training ammo to make me wary.
    Additionally, at a cop class some of their duty pistol ammo (5 rounds) failed to fire at all.

    There is a lot of voodoo in ammo, and i don't have access to the sophisticated equipment necessary to do any of it.
    So, for my peace of mind, one cycle is all it gets.

  5. #5
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    Quote Originally Posted by Paul Cataldo View Post
    Ok, then let me correct myself.
    I am NOT talking about light primer strikes. I am INDEED inquiring about the striking of the primer during chambering/dechambering...
    Please excuse the incorrect terminology...
    Piston operated AR's currently available all retain the same floating firing pin of their DI brothers so you would still get the dimple from chambering.

    Armalite has added a spring to the firing pin on the AR-10 as the pin is heavier and could cause a primer ignition with soft primers.

    Also note that there are harder primers available from CCI to reduce the effects of chambering with a floating firing pin.

    If you look closely at the design of the AR-15, it MAY be possible to get one to run (uncontrolled) full auto simply by increasing the mass of the firing pin.
    Perhaps a tungsten firing pin would do this.
    That's why the pin is kept to a pretty light weight.
    Randall Rausch
    AR15 Barrel Guru
    California Precision Rifle Club founding member

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    Quote Originally Posted by Snake RAH View Post
    Light primer strikes (not the marking of the primer from chambering) can be caused by:

    1) blunted firing pin or firing pin that falls below the minimum measurement on a firing pin protrusion gauge
    2) REALLY excessive headspace (another clue of excessive headspace would be wandering groups)
    3) hard primer
    4) deep seated primer
    5) weak or broken hammer spring
    6) improperly assembled hammer spring

    I've only had one gun with light primer strikes, and it was a M1 carbine with excessive headspace. I've shot a good amount of rounds through my AK clone, AR10, and AR15s, and have not experienced light primer strike.
    Actually a primer seated high can lead to this. For the priming compound to reliably detonate from the firing pin strike, the primer has to be seated firmly against the base of the primer pocket. If not seated against the base (i.e., "high"), the primer will protrude from the casehead and a portion of the firing pin's impact energy is wasted in seating the primer the rest of the way. You want that primer fully seated so all of the energy from the pin strike is transferred to the primer causing the compound to detonate.

    Another fault could be a crushed primer where the compound cracks underneath the anvil. This is caused by applying too much force during seating (or trying to prime crimped cases without first swaging/reaming away the crimp), crushing the primer compound. Many reloaders prefer a hand held priming tool for this reason, it allows them to feel the primer seat in the pocket. A press mounted tool, especially one that provides a lot of mechanical advantage, doesn't provide this same degree of feel and a ham fisted operator can crush primers without realizing it. They can still go bang, but if that compound in the primer cup cracks, it's more prone to flexing during the firing pin strike and not detonating.

    Lastly, primer contamination can kill a primer. While they are sealed, if oils or moisture gets into the compound, it can render them duds. Unless you are a reloader, you can't control any of these things, this is all up to the QC standards and production standards of the ammo mfg.

    I'm not a professional, so I don't have any protocol on monitoring my number of chambering cycles, but as Pat has pointed out, I have heard of cases where bullet setback occurs from repeated chambering. If neck tension still remains high enough you run the risk of increased pressures from a deep seated bullet. If neck tension decreases you run the risk of incomplete ignition and squib rounds. That's why crimping is so important, to prevent setback. The nose may get a bit deformed, but the round is still safe to fire. If you do get into the habit of recycling these "repeatedely chambered rounds" back to a position lower in the mag, you should be wary of impacting feeding/chambering reliability with a deformed nose/HP.

    If you have a set of calipers, I'd try to experiment with a round out of a lot of ammo. Measure the round before you chamber it the first time. Then chamber it, extract it and remeasure. Continue this and see if the overall length decreases. If it does, and if there's no tip deformation, your round is seating deeper into he case. If the tip deforms, you have a round that is either adequately crimped or has enough neck tension to prevent setback. You might want to bury that round into the mag, noting where you loaded it. Then fire away and make note on whether or not the round feeds reliably. If it hangs up you'll know your weapon is sensitive to nose profile and you can't get away with using these rounds for social purposes. Relegate them to range use only.

    Or do as Pat suggests and set an arbitrary limit on how many times you'll allow chambering of a round. That's a very safe, effective, conservative approach.

  7. #7
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    Thank you gentlemen,
    Pat your explanation was excellent, as is yours jmart. I appreciate the info, and everyone taking the time out of their day, giving me a chance to learn things such as this...

    BTW,
    The spring on the AR10 firing pin has me curious, and is very interesting. I wonder how well this spring works on the AR10's, and I wonder if one could be used on the AR15's????
    It's not the END of the world, but damn how I would love to end this issue entirely for good...
    Thanks guys...

  8. #8
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    Quote Originally Posted by Paul Cataldo View Post
    It's not the END of the world, but damn how I would love to end this issue entirely for good...
    You have failed to establish that there actually IS an issue, so how can you end it?
    Randall Rausch
    AR15 Barrel Guru
    California Precision Rifle Club founding member

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    Paul,

    I'm a little confused, in your first post, were you were talking about a failure to fire, then we go on to the little dimple in regards to chambering and chambering. For my clarification, when you said, "However, this was the first time that I've ever actually had a round NOT go off. - FTF. It was a quality round too. (75gr TAP FPD)." Are you meaning you had never loaded a round, then unloaded it without firing? Or did you have a FTF with an actual light primer strike? Or just a dud?

    Also, it is very hard to get a round to go off by chambering. I tried for a while and eventually gave up.

  10. #10
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    UVvis,

    I believe the discussion pertains to repeated chambering resulting in light strikes that eventually render a primer dead. I can see if enough light primer strikes occur that eventually when you yank on the trigger with the intent to fire off the round, the dimple in the primer could have got deep enough that the energy transferred is insufficient to detonate the primer. In effect, a Type 1 malfunction.

    Other contributors could be a faulty primer, either from improper seating or contamination. Or as Snake RAH pointed out, bad headspace, weakened hammer, heavy hammer, etc. Also, FWIW, bad headspace could be a mechanical problem with the chamber or a situation where the case is misformed. When the case is initially formed or resized, if the shoulder is pushed too far back you in effect have created an excessive headspace situation even though chamber dimensions spec out correctly WRT GO and NO GO gauges. This really isn't a headspace issue as much as it's a bad ammo issue, but the net effect is the same.

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