.375 Chey Tac neck clearance

Discussion in 'Reloading' started by LMK, Apr 1, 2009.

  1. LMK

    LMK Active Member

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    I am working on chambering reamer and resizing die dimension for a .375 Chey Tac based cartridge. Wall thicknesses are running about .014 to .0155 thick at the neck on a 10% sampling I mic'd. I intend to neck turn the brass to a fairly uniform .014 which gives me a case neck OD of .403 to .4035 with a 350 gr. SMK bullet seated.

    I am considering resizing die dimensions of -.0024 from chamber dimension at the base and a -.0035 sizing die dimension at the shoulder and .0035 sizing die dimension at the neck. I would like to hear an opinion on those clearances if anyone has one.

    The variables I am concerned about are brass springback and net clearance. if I cram brass into a hole that is .0035 smaller than the chamber what will I have as net clearance? For instance, if I assume .001 springback I would net .0025 of .0035 and is that enough in the neck area? Accuracy is a primary concern but I don't want a bullet that doesn't fit in 30 degree weather.

    Guidance is greatly appreciated.
     

  2. PFCSkoug

    PFCSkoug Active Member

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    I always order my resize reamers .003 under which gives you anywhere from .001 to .0015 which is plenty to load properly in most conditions but if you are going to but hunting in different weather conditions you might want to go with about .005 instead.
     

  3. davewilson

    davewilson Well-Known Member

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    the 2 main dimensions you have to worry about are the OD of the loaded round and ID of the chamber neck. if i were to build a 375, i would have a minimum of .004 clearance in the neck and .003 bullet squeeze.
     
  4. LMK

    LMK Active Member

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    Thanks Dave,

    I have been leaning toward slightly more neck clearance with a loaded round. I originally was considering something in the .0025 range for clearance but with such a large case and in hunting conditions that now seems too tight.

    I have a neck expanding mandrel, turning mandrel and neck turning tool coming next week from Sinclair. I have decided to get the brass cleaned up before I commit to the sizing die neck dimension.

    How can I calculate the springback on the brass once I know the loaded neck diameter. I assume there will be some as the brass is not dead soft.

    Any thoughts? I appreciate the input.
     
  5. davewilson

    davewilson Well-Known Member

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    most of us have been down the road you're heading. assuming we'll get better accuracy with a tight fitting round with very little clearance. i'm going through it now and i wish i'd never made a tight fitting neck. when you start turning necks, you'll open up a big can of worms that will probably end in less accuracy. unless you're building something to compete against Tony Boyer with, i would recommend not turning the necks, and putting a SAMMI spec chamber in your gun. most on here are after as much accuracy as they can get from a customized hunting gun and quite a few enjoy bench rest competition also. you just won't get any better acuracy turning necks and like i said, probably will not be as accurate because of the neck turning for one reason or another.
    maybe Kirby will jump in, he's played with the 375 and the Chey-Tac brass. i'm sure he can give you advice as far as springback, clearances, and anything you'll want to know about this round.
     
  6. LMK

    LMK Active Member

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    I am surprised to hear that as I have read numerous articles regardinging the benefits of neck turning. There seem to be a lot of true believers but I increasingly see "theories" that I question about precision shooting.

    Tight chamber theory aside, I still think uniform wall thickness makes sense purely from an engineering perspective considering bullet concentricity in the bore, uniformity of brass expansion/sizing, etc.

    I am going to go ahead with turning the necks on the brass I have as I am getting an average wall thickness deviation of .0012 to .0015 from one side of the case to the other. I may just stick to SAMMI clearances though. In reality, if the casing eccentricity is not interfering, the bullet should center the round as there is only .0005 clearance in the throat.


    I appreciate your input Dave and I'll let you know how things turn out.