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. For example, firearms using a simple blowback operation require bolts to be typically 1lbs/2lbs of weight.

Higher power cartridges require heavier bolts (typically 20lbs) to keep the breech from opening prematurely; at some point, the bolt becomes too heavy to be practical. For an extreme example, a 20 mm cannon using simple blowback and lubricated cartridges would need a 500-pound (230 kg) bolt to keep the cartridge safely in the barrel during the first few milliseconds.

Bolt thrust is not a measure to determine the amount of recoil or 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: