7mm Magnum Sizing

Discussion in 'Reloading' started by Full Auto, Jun 2, 2013.

  1. Full Auto

    Full Auto Well-Known Member

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    Question, I have been reloading for seems like 100 years and have still failed to understand sizing on belted magnums. I can get away with may be 4 loadings before having to toss my brass. I've some folks using a nickel to set the sizing die, also there used to be a company that sold a die specifically for belted cartridges, and allowed you to size all the way to the belt. Would appreciate a bit of input on how you all set the die. Thanks, ElDeelightbulb
     
  2. jimbires

    jimbires Well-Known Member

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    I pay no attention to the belt . I resize it as though it is a regular bottle neck case . all I do is measure my fired brass and set my sizing die to bump the shoulder back about .002 . you will need a tool to measure case length . once fired brass will probably not be expanded completely , so I only neck size it the first time . after the second fire the brass should be fully expanded and you can set your die up to size the brass to your chamber .

    so far I have not needed the belted case die .
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  3. Shootin4fun

    Shootin4fun Well-Known Member

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    How do you do that with a full length seating die? Are you saying that you just don't adjust it down all the way to touch the top of the case holder with the ram all the way up?? If so, doesn't that mean that you're not resizing the whole neck?
     
  4. Shootin4fun

    Shootin4fun Well-Known Member

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    Okay, so we see on the I T site that the proposal is to use standard FL sizing dies but adjust it to bump the shoulder by .002, and by using their headspace measurement tool to confirm your adjustment.

    But isn't it the case (no pun intended) that if you do it that way with FL dies, you're not actually (outside) resizing the whole neck? If its okay to do that, they is there any reason to buy neck sizing dies?

    Another question is, why bump the shoulder back .002? Is it to add some clearance to make cycling a round in more reliable?

    I think having the ability to checking overall length when setting the die after changing bullet types or lengths by using the headspace guage. Bullet length is more variable than bullet ogive, unless there's a design change.
     
  5. jimbires

    jimbires Well-Known Member

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    yes you are correct , your die will not touch the shell holder . your die should be sizing all of the neck . the die will size the neck until it touches the shoulder , then it will continue to size the neck while it pushes the shoulder back 0.002 .




    yes you should be resizing the whole neck , with the die adjusted this way . some guys still neck size only , I don't . I do have a few Lee collet dies , these only neck size , but then I also use a body die to push the shoulder back 0.002 . which is basically doing the same thing as using a full length die .

    bumping the shoulder back will ease cambering your ammo . if you neck size only , after a couple reloads the bolt will be hard to close on your rifle .

    yes I measure my COAL from the ogive . it is much more accurate than off the tip .

    redding makes the competition shellholder set , these are nice . I use these with a RCBS precision mic . it makes setting up your die easy . Jim

    Competition Shellholder Sets | Redding Reloading Equipment: reloading equipment for rifles, handguns, pistols, revolvers and SAECO bullet casting equipment
     
  6. Bart B

    Bart B Well-Known Member

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    When belted mag's were the norm for long range shoulder fired prone matches, the best scores were shot with new cases or fired ones full length sized all the way to the belt and setting the shoulder back a couple thousandths. Folks used to cut the body part out of a standard FL die then square its bottom to size the ridge in front of the belt back down all the way to the belt so it didn't interfere with the case shoulder slamming cleanly into the chamber shoulder perfectly centering the round when it fired. Such a die's called a "body" die for belted cases.

    Belted cases sized to headspace on their shoulders will center the case neck and therefore the bullet best in the chamber anyway. The firing pin drives the case forward hard into the chamber shoulder and that's what best centers the front of the case in the chamber.

    Larry Willis now has a collet die that's a lot better than a "body" die. Cases so full length sized can get 15 to 20 reloads with max charges easily.

    That aside, unless your and your hardware can shoot belted cases no worse than 1 MOA past 500 yards, that collet die may not make a difference. Just use a standard FL die and don't set fired case shoulders back too far. Seat bullets so they gently seat back a bit as they crunch into the rifling; that well centers even the crooked ones.
     
  7. Joe King

    Joe King Well-Known Member

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    This my method as well. I use this same process for all my rifle cartridges. I've never paid the belt any mind, it's one of those things that seemed like a good idea at the time but turned out to be more of a fruit blister than anything useful.
     
  8. Bart B

    Bart B Well-Known Member

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    In measuring neck runout on resized cases using neck only as well as full length dies (both with die neck diameters .002" smaller than a loaded round's neck), the full length ones better center the neck on the case shoulder. The body's held well in place as the neck's sized all the way down setting the shoulder back a bit.

    If the case neck's not centered on the shoulder on a case headspacing on its shoulder, both the neck and bullet will not be centered in the chamber throat.

    I believe all of this is why most benchresters finally switched over to minimal full length sizing their fired cases.
     
  9. Shootin4fun

    Shootin4fun Well-Known Member

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    I've never heard of hacking off the body of a FL die but at least for non belted cartridges it seems like that's the way to go for neck sizing. You relieve the pressure of a growing shoulder by always bumping it back, you size the whole neck, but you leave the body alone.

    So where do you cut the die- a few thousands past the shoulder? And how do you cut it precisely there?

    Considering you're just pushing the shoulder back and there is no die past that part of the shell, do you ever get some kind of rippling below the shoulder?
     
  10. Bart B

    Bart B Well-Known Member

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    I've cut a couple of 30 caliber belted magnum full length sizing dies off about 1/8" above the belt clearance groove and the same amount below the body-shoulder junction. After first using a standard full length sizing die on the belted cases setting the shoulder back a couple thousandths, I use this "body" die to resize the case body all the way back to the belt.

    That gets rid of the ridge that usually appears a few thousandths above the belt and sizes the body down to about new case diameter. Except for the case now headspacing on its shoulder, it's at virtual new case dimensions. Sub 3/4 MOA accuracy at worst 1000 yards down range providing all the other stuff in the rifle's good and put together right.
     
    Last edited: Jun 4, 2013