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Thread: Neck sizing vs. bumping the shoulder back a little

  1. #1

    Default Neck sizing vs. bumping the shoulder back a little

    While perusing another site I ran across a comment about how moving the shoulder back a couple of thousandths gives all "the benefit of full length sizing." Now I am fully aware that semi-auto & pump actions, need full length sizing to prevent chambering and extraction problems, but other than that what is the benefit? If the OAL for the case is within specs, and the OD dimensions of the neck are OK is there anything to be gained from moving the shoulder back even a couple of thousandths after it has been fire formed to fit the chamber? I have never had a problem with a neck sized case failing to chamber or chambering hard (I've been rolling my own since 1971), but I have experienced shorter case life with the full length resized cartridges for my 760 carbine, and I don't load these very hot compared to my other 06's. My groups almost always tighten up a little when I'm not using virgin brass in bolt actions and the Encore.

    Have I gotten it wrong all these years in believing that after firing a belted mag cartridge it will head space on the shoulder if you only neck size it, and that will help lengthen case life by slowing the stretching and thinning of the brass just in front of the belt? (After neck sizing I always use a Lee OAL guage and trimmer and usually I don't have to remove much, if any, material) I know that .002 isn't much but to quote Father Guido Salducci, "It all adds up." Why work your brass any more than absolutely necessary, and what is absolutely necessary?

  2. #2
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    Default Bump-Sizing vs Neck-Sizing..

    The biggest reason for sizing any of the case except for the neck, is becasue the chamber is not perfectly concentric. If a perfectly round chamber with a perfectly continuous shoulder angle and centered in the bore, then the only sizing we would need is the neck to hold the bullet.

    Perfect chambers are non existant and near perfect chambers are very expensive so we are stuck with something less than that. So when fired the brass comes out lop sided or egg shaped to the tune of a few thousandths of an inch, so we need to reshape it slightly. Also as brass gets work hardened with each firing, it fails to spring back to it's original pre-fired shape and we must reshape it particularly at the base and lower body. (This is the reason for the f/l sizing every third or fifth firing.)

    Bumping the shoulder back a couple of thousandths would also only be necessary every third firing or so and then only if the action did not have the strong camming action of the bolt action or the shooter just didn't want to fell that resistance. The action of rotating the bolt handle down will actually push the shoulder back to the amount necessary to allow it to chamber. In good concentric chambers I much prefer to do this as it insures good cartridge alignment in the chamber. This of course requires the bore and cahmber to be centered or any alignment will actually be "misaligned".

    Bump sizing is best done in a body die which will full length resize the case body and not touch the neck. It only re-establishes the minimum chamber headspace dimension by bumping shoulder back slightly. This minimum headspace dimension would only be established if the body die is screwed down to touch the shell holder. We would back it off to get to the one or two thousandths of set back that you mentioned. Redding makes body dies and a set of shell holders that are gauged in thickness increase of .002" steps to be used to get the right amount of shoulder set back for a particular chamber. This set of shell holders will decrease the shoulder to chamber headspace (push the shoulder back less) by the amount of .002" steps up to .010". This is the thickness of the shell holder and thicker is less shoulder bump. This allows us to adjust headspace of loads with a shell holder change. All of my f/l resizing takes place in these body dies and I adjust with the s/h to get leave the shoulder untouched.

    Redding also makes small base body dies for use in pumps and autoloaders that lack that strong camming effect of the bolt closure.

    Neck sizing dies are just the opposite of body dies and size only the necks. They would only set the shoulder back if the brass was from a from a rifle that was outside the SAAMI dimensions for the cartridge.

    Full length sizing or partial f/l sizing (with the die not touching the shell holder) will reshape the base and lower body that fails to spring back, for what ever reason, and will make the bolt closure easier. This usually makes the case grow longer and should be avoided until necessary. It also resizes the neck and pushes the shoulder back. In bolt action rifle I find full length resizing totally unnecessary in the first five firings and more with some calibers. A problem with the belted mags is that the case makers do not maintain that belt dimension very closely. If it is slightly oversized in diameter it will be a problem after the first firing and sizing doesn't help with belt diameter.
    Is there nothing so sacred on this earth that you aren't willing to kill or die for?



  3. #3

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    Murphy,

    Thanks for the explanation. My AAADD (Age Activated Attention Deficit Disorder) must be getting really bad because it's taken me longer than usual to conceptualize this. So, let's see if I get it. One of the benefits of minimum full length resizing is that it will align the bullet with the bore. This assumes that the chamber and bore are in perfect alignment and since nothing's perfect, any benefit will be dependent on the degree of imperfection in the bore chamber alignment. Until the cases work harden, a bolt action rifle has enough camming action to do the job with fired cases of correct length. Having the bullet aligned with the bore in the case does not guarantee perfect alignment "due to case or chamber eccentricities." (Hornady, 2007) So, in practical terms, with reasonable concentricity and alignment between the chamber and bore, and if you shoot a bolt action with adequate camming force, average Joe is not going to see any "benefit" from minimal head space sizing using a standard F/L die. The bolt handle might turn a little easier when closing the action. It would be better for accuracy and case life, when F/L sizing is necessary, to use a dedicated body die for F/L sizing and adjust it to maintain the head space established by firing, and then neck size. If I understood you correctly, a Redding small body die could be used the same way with non-bolt rifles to maintain minimal head space and not work the brass up into the neck lengthening the cases.

    Is this how the RCBS X dies work?

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    Wow! I do think you got it, in fact it even makes sense to me when you say it. Yes the body die is a better way to work it. Redding does now make die sets with the body die and the standard neck die and seater or with the new bushing neck die, body and seater die. These are only in the "most popular calibers", but can be bought individually for just about every thing. A body die is not so expensive and a guy can buy one for each of his hunting calibers and make better ammo.

    I think the RCBS X-dies are a sliding sleeve or mandrel type of die. It has a sleeve that floats up and down and stays with the case so that when the case is pulled out over the expander the sleeve keeps it in alignment and does allow the expander to overly stretch or deform the case mouth, this a common cause of cases needing trimmed. The expander sometimes overworks a case necks. The neck sizer or n/s portion of a f/l die makes the neck perfectly round and concentric the the expander destroys that when we yank it through the case. This stretches the neck length and deforms it some. This sliding sleeve is also used in the seater to maintain concentric alignment. The X-dies reduce this with the sliding sleeve and I think the new Hornady dies are similar to the RCBS X dies.

    Redding has this same concept with the micro adjustable, sliding sleeve, removable neck bushing die called the Competition Bushing Style Neck Sizing Die, or they have it without the sliding sleeve called the Type S- Bushing Neck Sizing Die. (All these upper case letters could be replaced with a $, they is pricey.) I have a few sets for specialty target calibers of the comp. bushing sizer and comp seater with a body die. This is the epitome of extreme precision for professional shooters who demand "perfect" ammo for an upcoming match (You can thank David Tubbs Kyle Brown for that) My PPC's and BR calibers and the 6.5 Scan, 6.5-284, 6.5-08 and 6.5-300 RSAUM, and a half dozen that start with 30 something, are in these. Redding also has a carbide expander button replacement for most calibers and it is round and slick and not only eliminates the need for lube it is round and doesn't deform case necks the way most expanders do. This and the body die are the most bang for the buck for us common shooters or for hunting ammo.

    Yes they have the small base body die for the M760, which will help.

    I think most companies are making these types of dies now, I am just more familiar with the Redding brand. I have many shining examples of their wares. I have a set of the new Hornady New Dimensions Dies for the 375 Ruger and they have a sliding sleeve in the seater die for better alignment. Any of these are to make finished rounds with less runout and better aligned ammo. I think they make better ammo than I am a shooter.

    Everything in this life is upside down. Back when I could shoot we couldn't get good barrels or dies and now with all this new stuff and high level precision it's all I can do to find the loading bench let alone the front sight. Youth is wasted on the young!
    Is there nothing so sacred on this earth that you aren't willing to kill or die for?



  5. #5

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    Murphy,

    Thanks again. This is starting to be come clear to me. I may order a small body die for loading the 760. This rifle is capable of shooting groups that defy what one would expect from a gun with the type of trigger (or more accurately firing pin release mechanism as those creepy crawly things should be called) it has. At the least, I should get a few more reloadings before I have to send the cases to the recycler.

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