chamber size, brass, neck turning and clearances

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by tstarky, Sep 24, 2012.

  1. tstarky

    tstarky Member

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    Sep 24, 2012
    I am reloading for a 30-06 old rifle and a 7MM RM new rifle, both Reminton 700's. This is the neck turning stage. I have used a Redding neck checker and will be neck turning the brass that has more than a 0.0015 variance. Both calibers case walls at neck are between .011-.016 thick. So if I turn to .012 to remove the variance (less than .0015) the following circumstance is:

    .307 = bullet OD
    .012 = case wall thickness side 1
    .012 = case wall thickness side 2
    .331 = reloaded bullet at neck
    +.002 = clearance to chamber wall (x2, .001 each side)
    .333 bullet + clearance to chamber wall wanted

    .3406 = chamber measurement
    -.3330 = bullet + clearance to chamber wall wanted
    .0076 = excess clearance to chamber wall ???

    Both the 30-06 & 7MM have near the same ".0076 = excess clearance". How will this "excess clearance" effect my accuracy? Do I need a new barrel or thicker brass? Or do I need to worry about the .0086 clearance for long range shooting as defined up to 1000yards?
     

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    Last edited: Sep 24, 2012
  2. Mikecr

    Mikecr Well-Known Member

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    #1 you can't measure neck thickness accurately with that setup.
    #2 with both those cartridges, you're gonna have so much runout and pushed/trimmed away brass from sizing the case bodies, that turning won't begin to help.
    There won't be ANY gains in neck turning these.

    You'd be better to get a neck mic, and cull your brass to use those ~15thou thick mid neck and with little natural variance.
    .308+.030 = .338, your fired brass from a .341NK chamber will springback to ~.340.
    This you can size with a bushing without over working necks, and since the thickness variance throughout the body length is consistant, your sized brass will stay straighter longer.

    As far as accuracy,, it will amount to what is sustained in the long run.
    That is, a working combination that is upset with mutated/overworked brass every 4-5 reloads, is not really a working combination.
    And if your 1000yd goal is based on competitive format, you might breeze through IBS equipment lists to see little to no 30-06s or 7RMs (for reasons stated).
     

  3. shortgrass

    shortgrass Well-Known Member

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    Lots of missing information. Are these factory chambered barrels or custom chambered? What are the neck diameters of these chambers? Are these intended for hunting or for the bench?
     
  4. tstarky

    tstarky Member

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    The brass is new Remington, once fired in my rifle. The rifle barrel is stock factory original Remington 700 model.
    Intention is long range hunting and getting this rifle as accurate as possible with the existing action, triger(Timeny), stock and barrel. Plans to bed the action and free float the barrel next week.
    Goal is to create the best Elk hunting rounds for this rifle. Focus for this thread is the brass neck size.
    Chamber size at neck for this 30-06 rifle is 0.3406".
    Concerned that the original brass wall thickness not "fat" enough to reduce clearance from neck wall to chamber after removing the variance in wall thickness to tollerance of .0015.
     
  5. Bart B

    Bart B Well-Known Member

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    If the resized cases headspacing on their shoulders have their necks well centered on the case shoulder and reasonably straight, it doesn't matter how much clearance there is between the case neck and chamber neck. The necks will center perfectly in the bore when the case gets driven hard into the chamber shoulder by firing pin impact.

    Using full length bushing dies may well produce the best resized fired cases. Use a bushing diameter about 2 thousandths smalller than a loaded round neck diameter. Set the fired case shoulder back about 2 thousandths. Excellent accuracy with both the '06 and 7RM has happened with these dies and techniques. The only other tool that helps belted cases is the collet die from Innovative Technologies - Reloading Equipment that sizes the fired case body all the way to the belt; conventional dies don't.
     
  6. shortgrass

    shortgrass Well-Known Member

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    Bart makes some valid points and suggestions. If you're still worried about case neck diameters why not try another brand of brass? If you're worried about consistancy, from case to case, try a different brand of brass. I shoot a couple of rifles that were chambered with reamers speced for "NO TURN NECKS" figured using 'premium' brass. As for the neck diameter of .3046", that you posted, you did make a chamber cast to find that dimention, didn't you? If I'm going for particular dimentions, I do it the other way around, by specing the reamer to the brass/loaded rounds. Most factory chambers from Rem./Sav./Win. ect., are on the generous side, made to accept any makers ammo. "Tight" tolerances are not in their plan,,, within SAAMI specs is. Good luck with you 'quest'.