Theory
A firearm cartridge case serves two functions. The most obvious role is to provide a container for the powder, bullet, and primer. Additionally though, upon firing, the expanding case seals the breech of the firearm to prevent burning powder gas from escaping rearward. Most cartridge cases are made of brass and are designed to expand under pressure to form a close fit with the sides of the chamber, and then contract so that the case can be removed. Centerfire cartridge cases are thicker near the base to prevent expansion into openings between the rear end of the barrel and the action where the cartridge case is not fully enclosed or supported by the chamber.
When a firearm has more headspace than the cartridge design anticipated, the closing of the bolt or impact of the firing pin may move the cartridge forward to leave space between the chamber face of the action and the base of the cartridge. Pressure of burning powder gasses expands the thinner forward walls of the cartridge case to firmly grip against the sides of the chamber preventing rearward motion. The thicker base of the cartridge case (or sometimes the primer) may then be forced back into available space at the rear of the chamber, causing the case to stretch. Where rearward movement exceeds cartridge design, the primer may rupture or the base of the cartridge case may begin to separate from the forward walls. Either type of failure may release high pressure gas and case fragments from the action of the firearm. These may cause burns, cuts, or eye injury to the shooter or persons standing nearby.
Headspace is a critical consideration in any firearm. It should be checked by a competent person using the appropriate headspace gauges prior to using the firearm.
Read more about this topic: Headspace (firearms)
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