15 Nov 19
“Headspace”
The term, “headspace,” refers to the distance from the part of the chamber that stops forward motion of the
cartridge, to the bolt-face.
That part of the chamber is sometimes called the “datum-reference.”
When used as a verb, “to headspace” refers to the contact created between this part of the chamber and the feature of the cartridge that achieves correct positioning as the cartridge is shoved forward into the chamber:
1 Rim
2 Shoulder
3 Case mouth
4 Belt
7.62×51, 5.56×45, 6.8×43, and most other modern military rifle cartridges “headspace” on the shoulder of the brass cartridge case.
One can only wonder how this will all work with “composite” cases!
Old, mostly obsolete, military cartridges, like the 7.62x54R, 303 British, 30-40 Krag, are rimmed and headspace on the cartridge’s rim.
Most revolver cartridges, 38SPL, et al, are rimmed and headspace on the rim.
Most rimfire cartridges, like the 22LR, are also rimmed and also headspace on the rim.
When headspace is too short, the bolt may not close, nor lock.
When headspace is too large, the cartridge case may rupture, or the round may not fire at all
Most straight-walled rifle cases need to be rimmed, as the rim provides a safe and positive datum point.
Most straight-walled, autoloading pistol cases (9×19, 45ACP) headspace on the case mouth, as this suffices for normal pistol cartridge pressures. However, such pistol ammunition cannot have a heavy crimp on the bullet, as this creates a uncertain datum point.
When metallic rifle cases were first introduced, most were straight-walled and rimmed.
With the advent of smokeless propellant, shouldered cases became popular, and the shoulder made a positive datum point, so rims could be eliminated, replaced with a flush “extractor groove” (a virtual necessity with the advent of box magazines).
Some, like the Russian 7.62x54R, retained the rim, but mostly due to bureaucratic momentum, not ballistic necessity.
“Belted” rifle cases are a relatively new development. The term refers to a shell casing with a pronounced “belt” around its base that continues several millimeters forward of the extractor groove.
It originated with British gunmaker, Holland & Holland (H&H) in the first half of the 20th Century, and is found on their powerful, big-game African cartridges.
H&H engineers calculated that the belt would provide more positive headspacing than the cartridge shoulder. They were particularly concerned with cartridges with shallow shoulders being shoved too far forward into the chamber.
The idea was subsequently picked-up by some American gun-makers and ammunition manufacturers, like Winchester and Weatherby.
This “belt” slowly became something of a “standard,” expected on “magnum” cartridges.
On most modern, heavily and sharply-shouldered big-game rifle cartridges that still feature a belt, that belt is probably unnecessary, but of course, long-past any chance of correction!
/John
