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Thread: Vid explaining & testing oversize headspace

  1. #1

    Vid explaining & testing oversize headspace

    After the issue of the dodgy ammo I've been trying to learn about headspace and found this vid of a couple of american chaps purposely expanding the headspace to show what happens.

    It's good for a noob and is quite funny when they end up hitting the bolt with a hammer to get it to fire, as the rifle keeps misfiring because of the excessive headspace they created!!
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  2. #2
    And here's one showing how to use 'Go - No go' guages
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  3. #3
    Great stuff.

    FWIW they obviously had misfire problems as headspace increased, but I suspect there were also other issues at play. On some other rifles the case would have been held back by the extractor claw so misfires would be less likely.

    By hammering on the cocking piece they were also pushing the bolt forwards so taking up some of the slack. However this would obscure the primer-flattening mechanism where the primers back out before the case stretches, than get pushed back in when the pressure drops and the case slams back into the bolt. The inertia of the hammer on the back of the bolt would have obstructed this.

    My experience of head separation is with .303, where the mechanism is similar but opposite.

    In .303 the case "headspaces" on the rim of the cartridge, not the shoulder. When the chamber is significantly longer and wider than the unfired case, which is how they were designed, the brass flows forward into the chamber. The end result is the same, it stretches at the weakest point, typically at the web of the head.

    Same applies to belted magnums, which were designed for same purpose i.e. loose chambers that absolutely must work in the worst conditions of dust/grit/mud/dinged ammo, controlled feed actions cycled in panic etc.

    No problem for military (one-time) use, but for reloaders it can be a pain. Separation after x3 FL sizes not uncommon. My solution is to to first do a weak fireforming load (10 grains Unique with lead bullet) then neck size.

    The .303 miltary chamber is very different from that of the unfired brass, I'll put up some comparison photos when I get the chance.

    The Lee Enfield was designed with this in mind, headspace (i.e. clearance from bolt head to the rim) can be adjusted by changing bolt heads among standard sizes using gauging, and when cases did separate (as they could) there was a special tool for removing the remains of the case. Must buy one when I next get the chance, they used to be commonly available.

    You could try the masking tape method on your unfired rounds to see what headspace they are working at (take out the firing pin first). I use scotch magic tape which is very uniform and uncompressible and cuts nicely with a scalpel.
    Last edited by Sharpie; 25-04-2013 at 23:16.

  4. #4
    Thanks for the info mate it's all a learning curve for me!
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