necking down 7saum to 6.5 gap

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by Tikkamike, Sep 6, 2018.

  1. Tikkamike

    Tikkamike Well-Known Member

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    I cant find 6.5 gap brass so wondering who has necked down 7 saum brass? Do you need to turn necks after in a standard chamber?
     
  2. trucraft

    trucraft Well-Known Member

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  3. clownbuster

    clownbuster Well-Known Member

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    Hi Mike,
    I sent you a response to your pm yesterday. I apologize, for some reason my response didn't send yesterday.
    Let me know if I can help.
    Thanks!
    Sanford Grasseth
     
    Last edited: Sep 6, 2018
  4. clownbuster

    clownbuster Well-Known Member

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    To answer your question, yes, you have to turn the necks afterwards. When sizing down brass, you're forming neck material from the shoulder which is thicker, which creates a thick spot, or "donut" at the newly-formed neck-shoulder junction that must be either reamed out from the inside of the neck, or expanded and turned off of the outside of the neck.
     
  5. Edd

    Edd Well-Known Member

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    It seems to me when you are sizing down, you would be forming the shoulder from neck material instead of forming the neck from shoulder material.
     
  6. clownbuster

    clownbuster Well-Known Member

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    No, it's the opposite. When necking down, (even to cartridges with the same shoulder length dimensions as in the 7 SAUM to 6.5 SAUM) you push the case into the die, the brass is squeezed and flows upwards/fowards, making the case longer and thicker at the new neck-shoulder junction from material used from the shoulder. It may be minute in the case of 7 SAUM>6.5 SAUM, but it exists and can be verified using pin gauges.
    To test it, grab a piece of 308 brass @ 2.025" and step it down to 6.5 Creedmoor. The length will be dramatically increased as the brass will flow forward, creating a much longer neck and a false shoulder formed from the shoulder material of the brass which must be removed.
    S. Grasseth
     
  7. Edd

    Edd Well-Known Member

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    That's not a good analogy.

    When forming 6.5 Creedmoor from 308 Win, you would be forming the neck from shoulder material.

    When forming a 6.5mm SAUM from 7mm SAUM, you would be forming the shoulder from neck material. These two cartridges don't have the same shoulder length. The shoulder on the 6.5 SAUM will be longer shoulder.
     
    Mikecr likes this.
  8. sable tireur

    sable tireur Well-Known Member

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    The Pat Sinclair thread is one of the best introductions to making up 6.5 RSAUM from 7mm RSAUM brass. He got it down after we made the first runs of GAP 4S from 7mm then had to switch to the .300 RSAUM when the 7mm well ran dry.

    Here is a utube video in case it might help a little more:



    It's not difficult to make these cases but it can get to be a little tedious when you want more than a hundred or so...o_O
     
  9. Deviant

    Deviant Well-Known Member

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    I just finished doing 100 Bertram 7 RSAUM down to 6.5 SAUM and I did indeed have to trim the false shoulder off. I trimmed to .0145 which didn't remove a lot from the neck but did from the new hump at the shoulder junction. I was worried at first that I was turning too far in but in reality I probably should go deeper. I may have to use a reamer and remove doughnuts after the first firing. I just didn't want to take a chance and ruin $170 worth of brass by going too deep since this was my first time necking down to a new caliber. It was a simple process with my Whidden full length sizer and a Sinclair Neck turning mandrel to open them back up before trimming to length and neck turning.

    Al
     
  10. clownbuster

    clownbuster Well-Known Member

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    Ok, we can accomplish this two ways. Neck it down, load it, fire it, and neck turn afterwards. Or form it and neck turn it.
    So when fire-forming shorter brass in a longer chamber, you don't think as the shoulder is blown forward any shoulder material will be made into neck?
    It's not clear what your point Edd, other than challenging my analogy, but regardless of your opinion and argument semantics, this issue exists when forming this brass. So when I said..."It may be minute in the case of 7 SAUM>6.5 SAUM, but it exists and can be verified using pin gauges..." myself, Pat Sinclair, my gunsmith, Dave Kiff of PTG, and thousands of others who have actually done this at nauseum are lying? Have you ever made this brass?
     
  11. Edd

    Edd Well-Known Member

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    Actually my point is clear. The point is, when you change the neck size from 7mm to 6.5mm without changing any of the other case dimensions, you don't turn shoulder brass into neck brass. You use neck brass to form the longer shoulder.
     
  12. Mikecr

    Mikecr Well-Known Member

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    Shoulder brass enters necks when FL sizing, lengthening necks with forming, or when necking up a cal. Necking down, in itself, does not do this.

    Also, I've found it easier to turn necks only with new/unsized cases.
    So in this plan you separate a few cases with flaws, like excess thickness variance, or differing in weight. Neck down a case as planned and measure resulting thickness.
    Now if you need to reduce thickness, turn the separated/still unsized cases +/- a schmidge, and then neck them down. Verify that one of the trial & error turned settings comes out to intended thickness after sizing.
    Now you know what to turn all new cases to -before any sizing.
     
  13. clownbuster

    clownbuster Well-Known Member

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    Ok I think I got it; when necking down, the shoulder will be longer, and the overall case will be shorter?
     
  14. Mikecr

    Mikecr Well-Known Member

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    No, case length does not change with neck sizing either way (up or down).

    Necking up a cal, with a mandrel, brings a bit of neck-shoulder junction into new neck. This is likely thicker than mean neck thickness, and is seen as a donut. But forming is never a done deal until fire forming is completed, and provided you never size donut area, it should not be a problem.

    Necking down, turns out some of the existing neck into new neck-shoulder junction. Nothing about this contributes to a donut.

    That is with NECK SIZING, which is not FL sizing.
    If you intend to form with a FL die, all prediction is gone, because FL sizing can change EVERYTHING (including case length).

    Another method, for radical conversions, is use of a pure forming die set.
    I've used a Lee Six forming set to create a 2 inch neck with a 30-06 case. This, just for fun. But like neck sizing alone, it does not change case length.
     
    Last edited: Sep 7, 2018