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    Peter Laidler's Avatar
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    Mk7 and others............ Why do the cases lengthen? You can only lengthen at/to the detriment of something else surely? And surely, if they DO lengthen, then lengthening in this context is simply stretching. And in that case they'd lengthen uniformly along the total length - yes?

    As I said I'm not and never have been a reloader but.......... Maybe there's more Q's than A's in the reloading game!
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    Quote Originally Posted by Peter Laidlericon View Post
    . . . Why do the cases lengthen? You can only lengthen at/to the detriment of something else surely? And surely, if they DO lengthen, then lengthening in this context is simply stretching. And in that case they'd lengthen uniformly along the total length - yes?
    Cases lengthen on firing when some degree of axial play exists between the bolt face and the rear surface of the cartridge base.



    1. The firing pin shoves the case forward, rim against the breech.
    2. The primer detonates. If it's not heavily crimped in place, it backs out, shoving the bolt and barrel as far apart as it can.
    3. The thin, forward part of the case expands to fill and grip the chamber while the bullet moves out of the case and down the barrel.
    4. The solid case head can't expand and grip the chamber, so it moves rearward, re-seating the primer, stretching the case walls just forward of the head, and stopping when it hits the bolt face. (In rear-locking actions like the Lee, the bolt and receiver also compress/stretch to add a little more movement. The higher the pressure, the more they move.)
    5. If (and only if) the amount of head movement exceeds the elastic limits of the case, the cartridge separates into two pieces. If it doesn't separate, it just get a little longer - and somewhat thinner at the point where it stretched. Image below shows this effect on the four US-made cases at right, all reloaded many times before being scrapped and sectioned.



    In reloading, cases lengthen during full-length sizing. Cases expand radially to fit the chamber on firing - how much depends on relative case and chamber dimensions, as well as chamber pressure. In the resizing operation, the lubricated case is forced into a chamber-shaped die slightly smaller than a minimum chamber. Since the case head is supported by the sizing ram and the case body is squeezed radially, the metal displaced from case body diameter moves in the only direction open to it - forward into the neck where the case is not constrained by the die.



    Both effects, localized stretch on firing and more general elongation during resizing, are cumulative during successive re-uses of the case. Elongation is addressed by trimming back the mouth of the case but stretching/thinning that occurs on firing is, for practical purposes, irreversible and eventually results in head separation unless the reloader monitors case condition carefully and discards those near fracturing.

    The whole business can mostly be avoided by sizing only the necks of fired cases, producing reloaded ammunition with zero-end play in the chamber where previously fired and that hasn't been significantly elongated by full-length resizing.
    Last edited by Parashooter; 09-08-2013 at 07:15 PM.

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