Headspacing on the Case Mouth

[40FIVER]

My reloading manuals mention the term "cartridges that headspace on the casemouth". Can someone explain this term in newbie reloader lanquage?

Thanks - Charlie

[KBCraig]

My reloading manuals mention the term "cartridges that headspace on the casemouth". Can someone explain this term in newbie reloader lanquage?

First, the very basics: "headspace" is a chamber dimension that is defined for every caliber. It's the distance from the face of the closed bolt, to a particular measurement point. It's important that it fall within specs, so that the round has enough room to chamber fully, but not so much room that the cartridge is too loose for safety.

For rimmed and belted cartridges, headspace is between the bolt face and the chamber face, or the area where the rim or belt sits.

For bottlenecked cartridges, headspace is between the bolt face and a defined point on the shoulder.

And for rimless straight catridges (meaning almost all pistol calibers), headspace is measured from the bolt face to the forward edge of the case mouth.

If your case is too long, the round won't chamber. Too short, and any number of things could happen, from not firing at all, to firing and developing excess pressure (which can range from "not good" to "very bad").

Kevin