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

Higher power cartridges require heavier bolts (typically ) to keep the breech from opening prematurely; at some point, the bolt becomes too heavy to be practical. For an extreme example, a 20mm 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 A Look at Bolt Lug Strength By Dan Lilja

where: Fbolt = the amount of bolt thrust Pmax = the maximum (peak) chamber pressure of the firearms cartridge Ainternal = the inside area (of the cartridge case head) that the propellant deflagration gas pressure acts against

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

where: π ≈ 3.1416 r = the radius of the circle

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.

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.Stolle Panda Bolt Stress and Deflection Analysis

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 (peak) 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 (peak) chamber pressure of the firearms cartridge, like a round that is chambered in an already very warm chamber that can result in cooking off (i.e. a thermally induced unintended firing).

Bolt thrust estimations for various pistol/revolver cartridges Chambering P1 diameter (mm) Aexternal (cm2) Pmax (bar) Fbolt (kgf) Fbolt .22 Long Rifle 5.74 0.2587 1,650 427 FN 5.7×28 mm 7.95 0.4964 3,450 1,713 9×19 mm Parabellum 9.93 0.7744 2,350 1,820 HK 4.6×30 mm 8.02 0.5051 4,000 2,021 .357 Magnum 9.63 0.7284 3,000 2,185 .357 SIG 10.77 0.9110 3,050 2,779 .380 ACP 9.70 0.7390 1,500 1,130 .40 S&W 10.77 0.9110 2,250 2,050 10 mm Auto 10.81 0.9178 2,300 2,111 .41 Remington Magnum 11.05 0.9590 3,000 2,877 .44 Remington Magnum 11.61 1.0587 2,800 2,964 .45 ACP 12.09 1.1671 1,300 1,517 .454 Casull 12.13 1.1556 3,900 4,507 .500 S&W Magnum 13.46 1.4229 4,270 6,076

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

Bolt thrust estimations for various rifle cartridges Chambering P1 diameter (mm) Aexternal (cm2) Pmax (bar) Fbolt (kgf) Fbolt 5.45×39mm 10.00 0.7854 3,800 2,985 .223 Remington 9.58 0.7208 4,300 3,099 7.62×39mm 11.35 1.0118 3,550 3,592 6mm ARC 11.20 0.9852 3,800 3,744 .303 British 11.68 1.0715 3,650 3,911 7.92×57mm Mauser 11.97 1.1197 3,900 4,367 7.65×53mm Mauser / 7×57mm 12.01 1.1329 3,900 4,418 6.5×55mm 12.20 1.1690 3,800 4,442 .30-06 Springfield / .308 Winchester 11.96 1.1234 4,150 4,662 7.62×54mmR 12.37 1.2018 3,900 4,687 8mm Lebel 13.77 1.4892 3,200 4,765 7.5×55mm Swiss GP 11 12.64 1.2548 3,800 4,768 .375 Holland & Holland Magnum / .300 Winchester Magnum 13.03 1.3335 4,300 5,734 6.5×68mm / 8×68mm S 13.30 1.3893 4,400 6,113 .375 Ruger / .416 Ruger 13.52 1.4356 4,300 6,173 .277 FURY (SAAMI specifications) 11.95 1.1216 5,516 6,187 .300 Remington Ultra Magnum 13.97 1.5328 4,400 6,744 .300 Winchester Short Magnum 14.12 1.5659 4,400 6,890 .338 Lapua Magnum 14.91 1.7460 4,200 7,333 .300 Norma Magnum / .338 Norma Magnum 14.87 1.7366 4,400 7,641 .300 Lapua Magnum / 7.62 UKM 14.91 1.7460 4,400 7,807 .375 CheyTac / .408 CheyTac 16.18 2.0561 4,400 8,224 .50 BMG 20.42 3.2749 3,700 12,117 14.5×114mm 26.95 5.7044 3,600 20,536

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

Bolt thrust

Bron en licentie