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Thread: Headspace off the case mouth

  1. #1
    SD Regular Greener Jim's Avatar
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    Headspace off the case mouth

    Why isn't this more popular? Gives the maximum calibre and case capacity for a given case.

    We have 450 Bushmaster, 50 Beowulf, a whole host of wildcats, and most semi-auto pistol cartridges. Why not more? Anyone have any ideas?
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  2. #2
    Headspacing off the case mouth is not common (or advisable IMHO).
    All "normal" cases whether rimmed or bottleneck headspace away from the mouth on an area that isn't erroded by the hot combustion gasses progressively when in use. Safer that way & likely to remain constant throughout the full life of a rifle.
    Headspace tolerances are much tighter than trim length tolerances so another reason to not measure from the mouth.

    Ian

  3. #3
    SD Regular Greener Jim's Avatar
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    I get what you mean. Pressure thing maybe? I say this as I know of 9mm browning hi-power barrels that have had loads of rounds through them and the barrels weren't replaced. A lot of that will have been fairly rapid fire as well. But 9mm is low pressure.

    I know it's a ballache but if the headspacing area did erode then it's a relatively easy job to re-tenon and chamber the barrel. You'd only have to take off a gnats pube.
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  4. #4
    Another thing that worries me is that some chump would still put on a hefty roll crimp and be completely unaware of the niceties required.

  5. #5
    Other than straight walled rimless cases, why would you want to?? Would you headspace a bottle necked case off of the mouth? Or a rimmed case? or belted magnum?? There would be no point: They already have easily established and maintained datum points for headspace. Headspaceing off of the case mouth would require a fastidious attention to case length that neither handloaders nor manufacturers wish to deal with other than in straight walled cases.~Muir

  6. #6
    SD Regular Greener Jim's Avatar
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    It's straight wall rimless I'm referring to Muir. It would take a lot of attention to case length and that is likely why it's not done more on factory cartridges. However, isn't case length just as important on the factory cartridges I mentioned in the OP?
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  7. #7
    I look at the fact that headspacing off the case mouth for rimless straight walled cases, is the only way to prevent forward travel of the case during ignition in magazine fed or single shot firearms, full moon and half moon clips prevent this when rimless cases are used in revolvers.

    Ian.

  8. #8
    SD Regular Greener Jim's Avatar
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    Exactly Ian. I guess my point is that 375 ruger has the same rim diameter as a standard belted mag, but the ruger cartridge has no belt. Instead it is base diameter all the way up this gaining case capacity despite being designed for 30-06 length actions. So what's stopping someone building a straight cased ruger big bore? Despite the expense
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  9. #9
    SD Regular Greener Jim's Avatar
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    Like this is an .510" calibre built on a RUM case
    Attachment 67850
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  10. #10
    Quote Originally Posted by Greener Jim View Post
    Like this is an .510" calibre built on a RUM case
    Attachment 67850
    Is there actually a rifle chambered for that .510"? lots of these extreme cartridges are just a made up exercise, however a straight walled bigbore based on the 375 Ruger is possible but consistent headspacing would mean regular case inspection and any necessary trimming to make sure that the case length stays within the given headspace specs, the .400" Wheelan based on the 30-06 had a barely visible shoulder and as such headspace problems were abundant making the unpopular amongst those who dared to try it.

    Ian.

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