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Importance of case head thickness relating to accuracy

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  #1  
Unread 04-06-2003, 07:52 PM
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Importance of case head thickness relating to accuracy

My 280AI is showing me she wants to group in the 2's and 3's,..BUT,every few shots she will sling one. The brass is Lapua, the cases are .001" neck runout (loaded) at most, even the foulers are only .002". I use a redding neck bushing die to size the necks down .003" and I only size about 60% of the neck. The primers are gold medal, the powders are RL22 and 7828. The bullets are SMK 150gr.

I decided to learn how to measure the case head thickness with my RCBS casemaster guage, and the directions say that if the needle stops from it's unwinding motion and reverses momentarily, that indicates an issue. Could this be my flyers shots???? or is this unrelated???

I am embarrassed to say that I have not tracked the number of firings on each case, and they are not all on the same firing either. I got this rifle along with new and pre-fired brass before I really got into the long range and reloading stages as far as I am now. The rifle will do .250", I have seen it before, what is happening here???

Thanks
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  •   #2  
    Unread 04-07-2003, 12:11 AM
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    Re: Importance of case head thickness relating to accuracy

    Hello,

    I was having a simular problem except I was full case sizing and my cases were getting too thin. As soon as I got new cases, it fixed it imediatly. I went from .100's - .300's to over 3" with worn out cases. I dont know if that is your problem, especially scince you neck size only, but give it a whirl, you may be suprised.
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    Unread 04-07-2003, 07:15 AM
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    Re: Importance of case head thickness relating to accuracy

    Michael, how many rounds/firings before your cases were garbage'd out?

    Thanks
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    Unread 04-07-2003, 07:32 PM
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    Re: Importance of case head thickness relating to accuracy

    .002" is not much, depending on how tight your chamber neck is, that may or may not do much. Primer pocket uniforming is good but you still have to be careful as to how much pressure you use in seating them. You can deform the inside with too much pressure. When you feel them want to stop, you should stop. Never "mash" them in.

    Have you tried primer flash hole de-burring? That can also have an effect.

    You are right about case volume. Most of the time, if cases are used off of the same lot and dont deviate more than 2 grain, they should work well.

    Are you checking your seating depths with a comparator each time you load a round while you are trying and eliminate this bug? Also are you abbsolutly sure your case necks are STRAIGHT?? The .25-.5" fliers sounds to me like crooked necks, which in turn hold bullets crooked.

    Also let me know what bullet you are using and get me dimensions of the, ie overall length, boat tail legnth, base diameter and nose legnth. And weight. Also what is the average velocity with this bullet?
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    Unread 04-07-2003, 07:35 PM
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    Re: Importance of case head thickness relating to accuracy

    I almost forgot, Are you turning your necks? also try .002 or even .001 neck tension. If you are not turning your necks, this could easily be the problem.
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    Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
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      #6  
    Unread 04-07-2003, 08:11 PM
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    Re: Importance of case head thickness relating to accuracy

    Michael, if it is OK I will e-mail the results and info as I can, to you. I will need to do some measuring to get you all that information.

    here are some basics

    -150gr SMK
    -cases all trimmed to 2.455"
    -necks with loaded round, neck run-out no more than .001" for grouping rounds, and an occasional .002" which I use as foulers and to get back into the feel of the trigger before I try groups.
    -I am using the lee autoprime, and have not tried to "feel" the seating pressure, maybe I should look into that more carefully.

    I don't have a comparator as of yet, but I do make sure all rounds are within a few thousandths (the points aren't always the same length, but I assume the seater grabs them at the same dia) The die is a redding seater and s series bushing neck die.

    sized neck is .310" and loaded neck is .313".

    Thanks for your help thus far, and please let me know if you see any clues with what info I have already provided.
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    Unread 04-07-2003, 09:44 PM
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    Re: Importance of case head thickness relating to accuracy

    Sure you can email your info to me.

    meichele@ratexenterpriseshome.com
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