How is the chamber defined?


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Lucky
December 21, 2006, 08:34 AM
Afraid to say it, but I don't know where the chamber starts. If you have a revolver, it's clearly the cylinder. But with other firearms does it include the area above the magazine?

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armedpolak
December 21, 2006, 09:18 AM
on an autoloader, when you insert a mag, athen rack the slide, where does the cartridge go? to the chamger. except the bullet is partially in the barrel. so i guess the chamber ends on the "step" and then the barrel begins.

JesseL
December 21, 2006, 12:00 PM
It's the area that contains the cartridge when the weapon is in battery.

On anything but a revolver that's basically a section of the breech end of the barrel approximately as long as the cartridge.

Being 'in battery' means the slide or bolt is completely forward and closed.

rockstar.esq
December 21, 2006, 02:59 PM
A cursory follow up on the earlier and well informed posts.

The cartridge headspace pertains to the chamber as well.
The breech end of the barrel has the profile of a loaded cartridge reamed into the inside diameter of the barrel. The short and unrifled area just in front of the chamber is called the "chamber throat" or leade. On rifles this area wears with shooting ultimately requiring barrel replacement. Having a loaded projectile too close to the throat can create excessive pressures however there are always exceptions to a rule.

The headspacing is accomplished in a few different ways. On some rimmed cartridges, the rim establishes headspace in the same way as a standard revolver. The belt on belted magnums essentially works the same way. On "rimless" cartridges the case is headspaced off the shoulder (if there is one) or on the case mouth (like the 9mm or .45ACP). In all cases the area identified for headspacing is called the case "datum". This is a critical dimension and should be maintained by case forming, trimming, or swaging.

Please note that the "case head" is the thicker area at the bottom of the case where the primer and extractor groove are located. The "case head" may or may not have much to do with headspacing. Often case head separation is an indicator of worn out brass, excessive pressures, or improper annealing.

ArfinGreebly
December 21, 2006, 03:05 PM
I have an M1 Carbine.

The .30 carbine cartridge is "rimless" but slightly tapered.

How is headspacing achieved here?

JesseL
December 21, 2006, 04:46 PM
The .30 carbine cartridge is "rimless" but slightly tapered.

How is headspacing achieved here?

Despite the slight taper, .30 Carbine is considered a "straight-walled rimless" round, and thus headspaces on the case mouth. So if you ever reload for your carbine it you will need to use a taper crimp, rather than a roll crimp with would adversely affect headspacing.

Lucky
December 21, 2006, 05:27 PM
Perfect, thanks.

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