Help On Understanding Neck Dimensions?

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by MAELTY, Sep 27, 2004.

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    MAELTY Active Member

    Mar 14, 2003
    The gun: Win Mod 70 7mmWSM (factory barrel, pillar bedded and tiggerwork done)

    Outside neck dimension of new, unfired cases: .316

    Outside neck dimension of fired case: .324

    Outside neck dimension after resizing using RCBS dies: .314

    Outside neck dimension when loaded: .316

    These numbers are an average of 20 cases.

    When using 162 AMAX's and 72 grs of H1000 the gun shoots .75" @ 100 on most days.

    1) Are these dimensions within the ballpark?
    2) When I use my Redding bushing die (.314 though they come out at .312 which Redding says can happen on there tech site) the loaded shells still measure .316 on the neck and the gun produces the same results.
    Am I using the correct bushing?


  2. Varmint Hunter

    Varmint Hunter Well-Known Member

    Dec 26, 2001
    No matter how you resize your cases the finished round will have the same neck diameter. This measurement is simply the neck wall thickness x2 plus the bullet diameter (.284").

    It looks like you have chosen the correct bushing, which is generally .002"-.003" less than the loaded round neck diameter.

    As you can see, your chamber will allow aprox .010" of neck expansion necessatating about .012" of resizing for reloading. This can shorten brass life a bit. A tight neck or minimum spec chamber reduces this problem.

    All your figures seem ok except that I'm surprised to hear that a .314 bushing could result in a case neck diameter of .312". If anything, brass spring back would result in a diameter of 1 or 2 thousandths larger than the .314" bushing.

    Have fun - It sounds like you've caught the bug. [​IMG]

  3. Fiftydriver

    Fiftydriver Official LRH Sponsor

    Jun 12, 2004

    Your numbers are running right with what you will find in most factory rifles these days. So for a factory rifle they are certainly within specs.

    I would say that your .314" bushing marked wrong. As Varmint Hunter points out, using a .314" bushing will generally result in case neck diameters in the .3145" to .315" range. I would say your bushing is undersized for its markings.

    That said, if you are loading in a factory chamber, which you are, this will not effect your accuracy at all compared to the other variables in the factory chamber such as the loose neck and loose throat I am sure.

    Just to give youan idea about the difference with a custom chamber and a factory chamber, if you came to me and asked me to build youa rifle in 7mm WSM, the first thing I would ask what you would be doing with the rifle.

    If you were big game hunting, I would sit down with either a dummy round or a sample of factory ammo and take dimensions off these rounds.

    I would have the reamer neck cut to 0.003" over loaded round diameter which would leave 0.0015" of bullet release when fired. This is about as tight as I like for a big game rifleyet it still offers greatly increased case life and accuracy.

    In this chamber your necks would expand 0.003" total in diameter compared to 0.008" in your rifle.

    To be honest, a factory rifle that hsa a neck diameter of only 0.008" over loaded round diameter is a pretty good one.

    The only thing wrong and its not really anything wrong, is that you have more neck tension then is really needed from your bushing necking your brass to .312".

    This will have no effect on accuracy or consistancy if your case mouths are square and properly deburred. Also, using the 162 gr A-Max with its long boat tail helps ease the bullet into the case neck.

    With a bullet of this design, a decent rule of thumd is that any load that will group 3/4 moa or under at 100 yards will look very good as you range increases.

    I have seen several loads with this A-Max and most other heavy A-Max bullets that shoot in this range at 100 yards and will group inside 1/2 moa at 200 yards and 1/3 moa at 500 yards.

    Try the load out at longer ranges and this will tell you everything you need to know about the load.

    It is vastly easier to tell a good load from a great load at 300 yards and out.

    Sounds like your on the right track. Your using good loading tools and and getting fine accuracy for a factory rig.

    As long as your bullet run-outs are low, I would not worry about getting another bushing.

    You may want to tweak the OAL of your ammo to see if this effects group size at all but to be honest, just step back to 300 yards at least and see how this load shoots as it, may suprise you!

    Good Shooting!!

    Kirby Allen(50)
  4. jb1000br

    jb1000br Well-Known Member

    Jul 8, 2003
    when taking a big bite with a bushing (.324-.312) in one step, the neck will ALWAYS be smaller then the bushing says.

    this is because of the great angle that the large case is entering the small bushing. teh angle continues thru sizing and therefore makes the neck smaller then it should be.

    here is the fix:
    get a .320 and .316 bushing -- size the cases in increments (fired-->.320-->.316-->final(.314?)

    step it down like this with the generous factory chambers and it will solve your problem. IOW the final sizing will match the bushing [​IMG]