• Factory ammo has to fit any rifle's chamber. There are specifications for how big the chamber for a given cartridge should be, but there are tolerances or "wiggle room" where factory chambers can be slightly bigger or smaller and still take factory ammo OK. This "one size fits all" factory ammo sizing is what full-length resizing gives you.

    This is handy to be able to do for several reasons:


    *Semiauto, auto, pump and lever guns need full-length sized cases at all times in order to feed correctly.

    *Brass that was fired in another gun should always be full-length re-sized, at least for the first firing.

    *As was mentioned above, neck-sized brass for a bolt action needs to be full-length re-sized every once in a while, in order to feed and chamber well


    Neck sizing:

    For a bolt action rifle, neck sizing does just what it sounds like... Only the neck is re-sized, leaving the body of the case as a close fit for the chamber of the gun it was fired in.

    This is also handy to be able to do for several reasons:


    *The bullet is better aligned with the bore, increasing the accuracy potential.

    *The cases are not compressed and then stretched so much by sizing and firing, so the cases last a lot longer.

    *Since the case is not being compressed by the die then stretched by firing, it needs to be trimmed for proper length a lot less often.


    So new ammo or brass should be neck-sized after firing the first time, and thereafter until it starts getting a little bit hard to chamber or after three or four reloads. Then you should full-length re-size the brass and check to see if it needs to be trimmed to proper length. After firing the full-length re-sized and trimmed brass, you can go back to neck sizing for a while.

    Note that some shooters, especially those who do not go for maximum loads find that they can neck-size for a lot more than just three or four times. How often you have to trim and full-length re-size your cases directly depends upon how hot you load your reloads.

    I've noticed that the reloading manuals I have do not explain any of this very clearly... - I dunno why.