Neck tension-- how much ???

Discussion in 'Reloading' started by GNERGY, Mar 17, 2010.

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    GNERGY Guest

    I was resizing some 338 Ultra mag cases today. I have both Hornady and Lyman dies and was checking the sized ID of the neck.
    Hornady .330 ID
    Lyman .333 ID
    So I was thinking the .333 ID would be less sizing and expansion of the brass and still have a good grip on the bullet to keep it from moving in the magazine.
    How much neck tension do you really need, or what is best?
    I have 2 rifles that are single shots so how much tension do you need for use or them?

    Thanks, Tarey
  2. Mikecr

    Mikecr Well-Known Member

    Aug 10, 2003
    On the single shots 1thou is enough.
    Otherwise I suggest you use no more than needed.

    10% variance in low tension is alot less than 10% variance in high tension..
  3. boomtube

    boomtube Well-Known Member

    Oct 8, 2007
    Good question, easy answer. Not simple to explain because it runs counter to what is commonly understood about bullet tension. I'll try.

    Basically, Mikecr is absolutly correct, "best" "tension" is 1 to 1.5 thou. In fact, it isn't physically possible to even have more than about 1.5thou.

    What most people call "tension" isn't. What they mean is how much smaller the neck ID is than the bullet, that's what machinests call an "interference fit", a jam-fit actually. Tension is NOT properly the amount of force needed to jam the bullet in, it's really the amount of pull required to take it back out.

    Our cases have enough elasticity, or spring, in them to allow them to retain a springyness that holds the bullets in place. But, if it's stretched too far the elastic of limit of the brass is exceeded and it just permantly stretches/expands. (That's what happens when we fire the rounds, the neck is "permanently" stretched too large to hold another bullet until we "permanently" squeeze it back smaller.)

    The elastic limit of common case neck brass is about 1 - 1.5 thou, rarely as much as 2 thou (it does vary a little by the physical size of the neck). If your .338 neck was sized and ball expanded to .330 your bullets will have to stretch it to .338 during seating. That much difference requires a noticible force to push them in and some feel that means it will hold the bullets tighter; it doesn't. You can prove that by seating, measuring the neck diameter, pulling the bullet and measureing it again; it will likely spring back only 1-2 thou from loaded diameter. Proving that no matter the neck diameter before seating, the permanent difference will only be 1 or 2 thousants (depending somewhat on how and where the measurements are taken and the original diameter of course). Thus, we can permanatly expand with an expander OR with a bullet but the end tension will be the same in either instance. There is no bullet tension value to expanding a neck to less than about 2 thou under bullet diameter.

    One significant factor in having lower seating tension (in your case that would ideally mean an expander of .336"- .337") is that less force needed to push the bullets in so they will enter more easily and, from that, tend to go in straighter.

    One rarely understood value of the Lee collet neck sizer die is it frequently achieves a proper REAL bullet tension fit.
    Last edited: Mar 18, 2010
  4. woods

    woods Well-Known Member

    Jun 4, 2006
    I agree with boomtube, I have done the test and measured with pin gauges accurate to within .001". I sized necks from a 264 caliber case with 3 different size mandrels

    mandrel size / # fired case / sized case pin size / pulled bullet pin size
    .262" mandrel / 5 times fired / .263" / .263"
    .262" mandrel / 1 time fired / .262" / .263"
    .261" mandrel / 5 times fired / .261" / .263"
    .261" mandrel / 1 time fired / .261" / .263"
    .260" mandrel / 5 times fired / .260" / .263"
    .260" mandrel / 1 times fired / .260" / .263"

    notice the pulled bullet pin size was the same. So I size all ID's to .003" below caliber to make sure I get consistant stretching of the brass and go from there. Other than crimping, I don't think you could get any more bullet grip even if you sized the ID down to .010" below caliber.

    Now there is another factor in bullet grip other than ID and that would be the surface interface of the bullet and the inside of the neck. Imagine the inside of the neck having multiple scratches and burrs as opposed to a completely smooth inside neck surface that was coated with a dry lubricant like graphite or mica. IMO there would be a difference in the amount of force needed to pull the bullet.
  5. Mikecr

    Mikecr Well-Known Member

    Aug 10, 2003
    Woods, yes frictional differences affect pull force. But this is not a factor.
    A neck fully expands on firing before the bullet has moved.

    What does affect tension in a meaningful(and useful) way, is the amount of neck sizing length. This can be useful for tuning with some component combinations.
    I start off with one cal of neck sizing at 1thou under(after springback).
    Just an easy standard for me, that's worked well w/regard to ES.

    Some day I'll play with it further(best load over wide temp range-vs-tension).
    But my main focus has been tension variance, as compared with an instrumented mandrel.

    LRHWAL Well-Known Member

    Jan 11, 2006
    Sorry to ressurect this, but I've been away and as I was hunting I figured you may forgive me...

    MikeCR, a question for you please and anyone else who wants to chip in...

    Okay, so I hear you, the neck expands before the bullet moves. The pull force would in my opinion (theoretically?) determine the "dwell time" or contribute to it. So if the internal neck surface creates more friction, then the dwell time may be a little longer. Also, the internal neck surfaces should ideally be pretty similar to ensure consistency in performance. Your thoughts?

    As an aside, I don't suggest that a smooth surface has less friction than a slightly rougher once cleaned with a brass brush for example as opposed to one that's polished, as the surface area actually contacting the bullet jacket may be less with a rougher surface, but this isn't relevant for my purposes, so I haven't really thought about this much.

    Thanks for sharing your thoughts.


    MTBULLET Well-Known Member

    Jul 16, 2007
    This is like overthinking the sun rising in the east, it really dosn't matter as long as it's CONSISTANT.
  8. new shooter

    new shooter Well-Known Member

    Mar 19, 2006
    So witch one would used if you were me for my 7mm stw 0.002 or 0.003 for lower ES. Thanks for your input. Joe
  9. Mikecr

    Mikecr Well-Known Member

    Aug 10, 2003
    Start 3thou - 1.5thou sprgbck, for ~1.5thou tension on ~.280" worth of neck.
    I think you'll find this is sufficient bullet grip, while providing consistent seating pressures and resulting seating depths.
    You might even give 2thou under a try, as the lowest tension you can get away with produces the lowest tension variance. But that don't guarantee lowest ES(there are more factors there).
    For your load you might need high tension. This you experiment to by varying the depth from .280 to maybe .300(+), or .250(-) for less tension.
    And then, it can be a double edge sword in that running high tension, invokes annealing more often, or tension variance will grow to the point of diminished returns(climbing ES, opening groups).

    I suspect this is more of a tuning aid for those who are not jammed.