Bolt thrust

Bolt thrust or breech pressure is a term used in internal ballistics and firearms (whether small arms or artillery) that describes the amount of rearward force exerted by the propellant gases on the bolt or breech of a firearm action or breech when a projectile is fired. The applied force has both magnitude and direction, making it a vector quantity.

Bolt thrust is an important factor in weapons design. The greater the bolt thrust, the stronger the locking mechanism has to be to withstand it. Assuming equal engineering solutions and material adding strength to a locking mechanism causes an increase in weight and size of locking mechanism components.

Bolt thrust is not a measure to determine the amount of recoil/free recoil.

With a basic calculation the bolt thrust produced by a particular firearms cartridge can be calculated fairly accurately.

where:

Cartridge case heads and chambers are generally circular. The area enclosed by a circle is:

where:

Equivalently, denoting the diameter of the circle by d.

A practical problem regarding this method is that the internal case head diameter of a particular production lot of cartridge cases (different brands and lots normally differ dimensionally) can not be easily measured without damaging them.

A complicating matter regarding bolt thrust is that a cartridge case expands and deforms under high pressure and starts to "stick" to the chamber. This "friction-effect" can be accounted for with finite elements calculations on a computer, but it is a lot of specialized work and generally not worth the trouble.

By oiling proof rounds during NATO EPVAT testing procedures, NATO test centers intentionally lower case friction to promote high bolt thrust levels.

...
Wikipedia