Bolt thrust orbreech pressure is a term used ininternal ballistics and firearms (whether small arms or artillery) that describes the amount of rearward force exerted by thepropellant gases on thebolt orbreech of afirearm action orbreech when a projectile is fired. The appliedforce has bothmagnitude anddirection, making it avector 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 ofrecoil orfree recoil.

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

${\displaystyle {\overrightarrow{F}}_{bolt}=P_{max}\cdot A_{internal}.}$^[1]

where:

• F_bolt = the amount of bolt thrust
• P_max = the maximum (peak) chamberpressure of the firearms cartridge
• A_internal = the inside area (of the cartridge case head) that thepropellantdeflagration gas pressure acts against

Cartridge case heads and chambers are generallycircular. Thearea enclosed by a circle is:

${\displaystyle Area=\pi r^2\approx 3.1416\cdot r^2.}$

where:

• π ≈ 3.1416
• r = the radius of the circle

Equivalently, denoting the diameter of the circle byd.

${\displaystyle Area=\frac{\pi d^2}{4}\approx 0.7854\cdot d^2.}$

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.^[2]

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

Instead of using the internal case head diameter, the external case head base diameter can also be measured with acaliper ormicrometer or taken from the appropriateC.I.P. orSAAMI 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.

${\displaystyle \overrightarrow{F_{bolt}}=P_{max}\cdot A_{external}.}$

where:

• F_bolt = the amount of bolt thrust
• P_max = the maximum (peak) chamber pressure of the firearms cartridge
• A_external = 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 (peak) chamber pressure of the firearms cartridge, like a round that is chambered in an already very warmchamber that can result incooking off (i.e. a thermally induced unintended firing).

The P1 (cartridge case base) diameters and P_max used in the calculations were taken from the appropriateC.I.P. data sheets.

The P1 (cartridge case base) diameters and P_max used in the calculations were taken from the appropriateC.I.P. data sheets.

• Blaser R93 Unfälle - zur Verschlußbelastung(in German)
• Zylinderverschluß - Verschlußkräfte(in German)

• Firearms
• Firearm actions
• Firearm terminology
• Ammunition

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