Bolt thrust


Bolt thrust or breech pressure is a term used in internal ballistics and firearms 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 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 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.

Calculating bolt thrust

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

Formula

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 can not be easily measured without damaging them.

Friction effects

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.

Practical method to estimate bolt thrust

Instead of using the internal case head diameter, the external case head base diameter can also be measured with a caliper or micrometer or taken from the appropriate C.I.P. or SAAMI cartridge or chamber data tables and used for bolt thrust estimation calculations.
The basic calculation method is almost the same, but now the larger outside area of the cartridge case head is used instead of the smaller inside area.
where:
  • Fbolt = the amount of bolt thrust
  • Pmax = the maximum chamber pressure of the firearms cartridge
  • Aexternal = the outside area of the cartridge case head
This method is fine for getting a good estimate regarding bolt thrust and assumes an overly large area that the gas pressure acts against yielding pessimistic estimations, generating a safety margin in the process for worse case scenarios which can result in increased maximum chamber pressure of the firearms cartridge, like a round that is chambered in an already very warm chamber that can result in cooking off.

Bolt thrust estimations for various pistol/revolver cartridges

The P1 diameters and Pmax used in the calculations were taken from the appropriate C.I.P. data sheets.

Bolt thrust estimations for various rifle cartridges

The P1 diameters and Pmax used in the calculations were taken from the appropriate C.I.P. data sheets.