Importance of case head thickness relating to accuracy

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by JustC, Apr 6, 2003.

  1. JustC

    JustC Well-Known Member

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    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
     
  2. Michael Eichele

    Michael Eichele Well-Known Member

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    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.
     

  3. JustC

    JustC Well-Known Member

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

    Thanks
     
  4. Michael Eichele

    Michael Eichele Well-Known Member

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    .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?
     
  5. Michael Eichele

    Michael Eichele Well-Known Member

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    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.
     
  6. JustC

    JustC Well-Known Member

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    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.
     
  7. Michael Eichele

    Michael Eichele Well-Known Member

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  8. Michael Eichele

    Michael Eichele Well-Known Member

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    About 10-12. Keeping in mind that was full case sized. 3 of them actually split on low pressure loads. Even the shooting session before these slit, the accuraccy had got real bad. Why the accuracy had got bad just because they were thin, I do not know. But I do know that it quit as soon as I got new cases. If you are neck sizing, your cases should last a long time, unless the neck in your chamber is really big. If you are getting just an occasional flier, it could be somthing else too. The list for possibilities could be huge. More common than not though if the rifle shoots very good and loads are fine tuned and components are in good shape, most fliers are caused by the shooter. Sometimes if there is a minor stress point in the barrel, when it reaches a certain temprature it will throw it off. This would of course change as the starting temp of the barrel changed. Keep in mind too that when a barrel warms up, it becomes more intollerant of small mistakes. A little too much preassure on the grip with your middle finger with a warm barrel can cause a right hand flier if you are right handed. I dont know how long your barrel is either, but somtimes if it is a little too short, the muzzle pressure smacks the bullet around on its exit and if you do everything perfectly it will shoot good, if make even minor mistakes, this will affect your performance. Also how far away are your fliers? and how often do they occur? also do they go in a certain direction? Let me know.
     
  9. JustC

    JustC Well-Known Member

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    Flyers are inconsistent in direction. they probably fly 1/4"-1/2" away from the group hole.

    My barrel is a 27 1/2" pacnor finished at .875" with a 9 twist, 11 degree crown etc. the action is trued and skim bedded into an HS Precision stock, with the trigger prfessionaly done by the smith. The barrel channel is releived nicely as well. The groups have been shot from a 15" bull bag and rear bag set-up as well as a windage adj front rest with rear bag. I also had a friend who has won the 1000yd match on occasion, at Quantico shoot it, with the same results. My cases or loads have to have something to do with it, if I were to guess.

    I do know that my body die, adjusted all the way down, won't push the shoulder back any. It came from redding too long I guess. Could not pushing the shoulder back .002" be a cause?

    I have been told that internal capacity fluctuations will cause flyers and that I should do the water test to determine any culls. What do you think of that suggestion? Also, was told that improper primer seating depth will yield flyers. I run the RCBS uniformer in the pocket every loading,......is there some trick or method I am unfamiliar with, related to pocket uniforming that is other than running the tool in and twisting until the pocket is clean and shiny?

    I don't think a single shell has more than 10 firings on it, but could be wrong. My powder charges, started at 57.5, 58.5, and 59gr respectively, with the RL22 (all WELL under max) for the 280AI in the Nosler no5 manual, and the 7828, if I recall was 58.5,59.5,and 60gr respectively, all for 5 shot groups. These are 2gr or better under recommended max, but close to max in the nosler no4 manual [​IMG]
     
  10. JustC

    JustC Well-Known Member

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    will do,..thanks.
     
  11. Brent

    Brent Well-Known Member

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    JustC,

    Are your cases from the same lot?

    One thing I see you can do to eliminate the case capacity as the cause is to trickle all the charges and use cases that shot into the same hole while setting the cases that produced fliers aside, you may have to shoot them again if you haven't already set aside the flier cases though. If the "good" cases produce fliers in the same fashion and regularity after this, you will be certain case capacity differences have nothing to do with it. If the "good" cases produce nice consistant groups you can look the culls over to find the source of irregularity that is causing the fliers, measure water capacity, neck thickness variations between cases and around the neck on the same case, RO etc, etc.

    Bullets: Try another bullet like the A-Max to see if it produces the same fliers. A good comparetor to compare ogive length and weighing them also might be necessary if it leads to the bullet being the issue.

    Turning a thou or so off the necks to clean them up would leave you with a bit less neck tension which is all you should need for work at the bench or if you're single feeding them. I'd only waste the time to do this on new cases from the same lot if it came down to being the cause though.
     
  12. Guest

    Guest Guest

    1) Ogive to base differences

    2) Neck tension differences

    3) Flash Hole not consistent.

    4) Primer not seated consistently

    5) bedding issue

    6) Scope or mount issue