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Thread: Seat depth for Primer

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    Seat depth for Primer

    How to determine the proper seat depth for seating primer?


    Do any cons of seating primer too deep?



    I normally seating my primer just deep enough for the end of primer to square with case based.

    But found some forums they recommend to seat primer to touch the flash hole of case. With their suggest, I can feel that the end of primer was deep into the primer pocket.

    Will variation of seating primer can effect on accuracy?
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    Re: Seat depth for Primer

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    Re: Seat depth for Primer

    Originally Posted By: Golfy Sniper
    Do any cons of seating primer too deep? Will variation of seating primer can effect on accuracy?


    The primer is intended to be seated at the bottom of the primer pocket which means it will be a little (~.004) below the case base. Some like to say install it until you feel the feet bottom out and then preload it a bit until the cup touches. If it is not seated all the way, you do risk some induced inaccuracy as the firing pin could seat the primer the rest of the way in the process of firing. In a rifle such as an AR, you increase the risk of slam fires if the primer is not seated all the way. With your bolt action, not so much a problem so long as it is flush or deeper other than possible induced inaccuracies.

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    Re: Seat depth for Primer

    I was taught to seat them 1. untill they stop and 2. they should be a few thousandths below the case head level. Again the way I was taught, you can feel them just slightly below. You shouldn't be able to get them too deep unless too much material has been removed from the primer pocket. In which case would most likely weaken the case to the point of failure and your firing pin may not extend far enough past the bolt face to strike the primer with enough depth and force to set it off.

    But unless you somehow found primers that were not tall enough(I don't know how....) or removed more material from the pocket floor than necessary, you should be able to seat it firmly against the floor or flash hole and move on to the next.

    Maybe I am wrong, but I seat all of mine firmly and a fuzz more. Which usually deforms the primer slightly, but not enough to set it off. I do it all by feel.

    As far as variances in accuracy, I would doubt it would be noticeble unless you are shooting Benchrest and tracking everything.

    Just my opinion

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    Re: Seat depth for Primer

    The primer should be seated to bottom out on the pocket regardless of depth from the head. IMO, consistent pressure after the anvil touches the pocket will give best results. Not seating the primer to touch the pocket is probably the most common cause of misfires in handloads.

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    Re: Seat depth for Primer

    Quote:
    In a rifle such as an AR, you increase the risk of slam fires if the primer is not seated all the way.

    I've heard this, but I don't understand why it would be true.

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    Re: Seat depth for Primer

    Its also why many people, including myself, prefer to use a hand tool for better feel when seating primers. The mechanical advantage and linkages makes it harder to tell how the primer is seated on a typical press.

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    Re: Seat depth for Primer

    Could you please see my pic.

    Do this one is too deep or not?





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    Re: Seat depth for Primer

    Originally Posted By: Golfy Sniper
    Could you please see my pic.

    Do this one is too deep or not?





    Really hard to say without a scale to measure by. I have had issues with primers that appeared the same. Happened with cases that I uniformed the primer pockets on and the depth collar slipped.

    Take the end of your caliper and measure the depth of the primer. Most agree that a min of .002 to around .005 is correct provided the primer is fully seated in the pocket.

    I now use a Redding primer pocket uniforming tool which is ground to the correct depth. In order to seat a primer too deep now I literally have to crush it with the priming tool.
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    Re: Seat depth for Primer

    MtnCreek,

    ARs, as well as most other automatic US martial arms such as the M1 and M14 utilize what's known as a "floating" or "inertial" firing pin. In practice, the firing pin merely rests in the bolt, and is not spring loaded in any way. The hammer strikes the rear of the pin, slaming it into the primer, and creating discharge. Otherwise, the pin just "floats" in the bolt, neither pushed to the rear, nor the front, by any sort of spring. As a result, when the bolt goes forward, the pin continues via inertia, and will actually impact the primer. The blow is light enough that it doesn't detonate the primer (if all is in adjustment and the ammunition is correct), but it does leave a small but quite visible dent in the primer. Try it sometime; chamber a round in any of these rifles, then extract the unfired round and look at the primer. The firing pin impact will be plainly visible. As you can imagine, anything that deviates from the correct seating of a primer, such as having it too high in the primer pocket, can drastically increase the chance of slam-firing. Dittos for too soft a primer cup, which is why primers such as the Federal Match primers (205Ms or 210Ms) are generally not recommended for use in Service Rifles. There are in fact several primer that are made with harder cups, specifically to duplicate the primers used in military ammunition. The CCI #41 and #34 are two such examples. Yeah, there's a reason for them, and that's why you want to seat primers fully below the head for use in such semi-autos. Serious safety issue here.
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    Re: Seat depth for Primer

    Golfy,

    Holding the case between your middle fingers and thumb, take the tip of your index finger and run it across the primer. You should feel a distinct "dip" as you cross the primer. So long as you can feel this, you'll be fine.
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    Re: Seat depth for Primer

    I see what youíre saying, but in my simple mind, the primer being high reduces the chance of it firing, whether intended or from the firing pin hitting it on bolt closure. Typically, a high primer will be seated with the first strike of the firing pin and will fire on second strike. But Iím only getting information from the high primers that didnít go off; any high primers that fired were not identified as high primersÖ

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    Re: Seat depth for Primer

    Entirely aside from the floating firing pin, it also means that the face of the bolt is slamming into the primer, without the support of the rest of the case head. Primers don't like getting slapped around like this, and sometimes take it very, very badly. In some instances no doubt, the primer will be "seated" by the bolt slamming into it, in others, you'll get a detonation. Either way, it's easy enough to avoid via proper primer seating the first time around.
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    Re: Seat depth for Primer

    This really isn't difficult...

    Seat them as deep as they can be.

    They need to wind up sub-flush with the casehead. If they're only flush or if they're proud, thats a problem.

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    Re: Seat depth for Primer

    Seated properly the anvil inside the cup will preload the priming compound. This is essential for consistent combustion.
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    Re: Seat depth for Primer

    I too have only used hand priming tools to date, with no issues. However, I'm interested in trying the priming functionality on my Forster Co-Ax that just got here.



    From reading how they set it up, it seems a little more precise and uniform. You still wouldn't get the 'feel' though, but it looks like they make it 'fool-resistant' on the depth issue.

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    Re: Seat depth for Primer

    BTW...Kevin Thomas nailed it on semi autos