Chamber dimensions for a fitted no-turn neck

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by LRHWAL, Sep 12, 2008.

  1. LRHWAL

    LRHWAL Well-Known Member

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    A question for my future planning so please bear with me and provide any input possible.

    My question is what is viewed as preferred clearance over loaded round diameter for a "no-turn neck" chamber?

    Here's what I "understand", but please can those of you with the knowledge comment on this?

    Clearance seems by most accounts to need to be 3-4 thou (although those who monitor closely, or neck turn for tight necks will go much tighter obviously). Is that a good number to use for "no neck turn" clearance?

    Example:

    Currently my 300 WM is a case in point. With WW brass my loaded round OD is around 0.333-0.334 (case walls are 0.0125-0.0129"). On that basis would a no-turn chamber be cut at say 0.337-0.338 (assuming I cut a chamber specifically for that WW brass)?

    I can't find the SAAMI specs on the net, but Forster on their "Outside Neck Turning Accessory Instructions" page lists them as 0.3397 ("+0 or -0.008" ", whatever that means as it's a pretty wide spread, so that seems to make no sense to me). That's strange to me too as my fired case OD measure 0.340 and there must be some springback in there, making the chamber larger than that (I haven't taken a chamber cast yet and of course it may be outside spec).

    What would SAAMI chamber OD in the neck area be for say the .300 WM and .243 Win?

    Finally, if I wanted to go to thicker brass for the 300 WM, what brands are likley to be closer to say 0.014" or so? any ideas?


    Thanks.

    WL
     
    Last edited: Sep 12, 2008
  2. Mikecr

    Mikecr Well-Known Member

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    SAAMI for 300WM BRASS is .3397 ND all the way.
    SAAMI CHAMBER neck is .3407 at the mouth, and .3421 at the neck-shoulder.
    This is from QuickDesign and RCBS Cartridge Design.

    This correlates pretty well with your .340 after springback, depending on where you're measuring.
    Does your fired brass come out with tapered necks?
    Will you be ordering your own reamer?

    0.0129*2+0.308 = .3338 (.334)
    A couple thou is enough clearance so .336 would work or .338 if you feel you need that much(mixed brass).
     
    Last edited: Sep 12, 2008

  3. BountyHunter

    BountyHunter Writers Guild

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    In my experience .0125 to .129 is really on the thin side for 30 cal brass in the case necks. I think you will run into way more over than meeting or under those specs. Most will be in the .0140 to .0155 on the high side.

    Plus a .0120 neck is really thin and subject to damage. You rarely see magnum tight necks cut that under .013

    I know I and a couple smiths spent some time measuring the WSMs and decided on a .340 no neck turn reamer.

    Better look at more than one lot of brass and do a lot of measuring before you decide to go under .340 IMO IF you want to not neck turn

    in 1k BR, a .336 is a tight neck gun! We normally run .002-.003 for clearance.

    I would also strongly recommend that you order your own reamer $150 and under and will do many barrels. PLus if you talk with the reamer manftr, tell him you want no neck turn and minimum SAAMI specs.

    BH
     
  4. LRHWAL

    LRHWAL Well-Known Member

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    Thanks for the replies Mike, BH. That was exactly the kind of input that I was hoping for.

    In an exaggerated way, the neck are a little like this ( ) - i.e. largest diameter is neither at the case mouth, nor at the neck shoulder junction. By the way, rifle shoots good, but I'm wrestling with runout issues.

    Yes BH, compared to the 308 brass that I've measured the 300 WM is really thin. I was surprised too. Winnie as I said and RWS basically the same, although I'll check again tonight. I also have a few Winnie cases I'll not use as I picked them up - will check those too.

    Domestic PMP is a lot thicker, but the cases are very inconsistent around the necks and I won't be using them.

    For interest I'll post a question as to neck wall thickness under the "reloading" topic.

    Yes, I'll measure a bunch thanks and have some buddies check on their micrometers too. That's good advice.

    WL
     
    Last edited: Sep 15, 2008
  5. LRHWAL

    LRHWAL Well-Known Member

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    Thanks again for the replies and input.

    I'm considering getting a custom reamer fitted to WW or RWS brass.

    Will probably go for 3 thou clearance so I won't need to turn and measure each load.

    WL