Bolt thrust

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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.

Contents

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 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

[1]

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.

The green line denotes the internal case head diameter and the red line the external case head base diameter of a rifle cartridge case. Bolt thrust.JPG
The green line denotes the internal case head diameter and the red line the external case head base diameter of a rifle cartridge case.

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. [2]

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:

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

ChamberingP1 diameter (mm)Aexternal (cm2)Pmax (bar)Fbolt (kgf)Fbolt
.22 Long Rifle 5.740.25871,6504274,268 N (959 lbf)
FN 5.7×28 mm 7.950.49643,4501,71316,794  N (3,775  lbf )
9×19 mm Parabellum 9.930.77442,3501,82017,847  N (4,012  lbf )
HK 4.6×30 mm 8.020.50514,0002,02119,816  N (4,455  lbf )
.357 Magnum 9.630.72843,0002,18521,428 N (4,817 lbf)
.357 SIG 10.770.91103,0502,77927,248 N (6,126 lbf)
.380 ACP 9.700.73901,5001,13011,085 N (2,492 lbf)
.40 S&W 10.770.91102,2502,05020,101 N (4,519 lbf)
10 mm Auto 10.810.91782,3002,11120,701 N (4,654 lbf)
.41 Remington Magnum 11.050.95903,0002,87728,213 N (6,343 lbf)
.44 Remington Magnum 11.611.05872,8002,96429,069 N (6,535 lbf)
.45 ACP 12.091.16711,3001,51714,879 N (3,345 lbf)
.454 Casull 12.131.15563,9004,50744,197 N (9,936 lbf)
.500 S&W Magnum 13.461.42294,2706,07659,584 N (13,395 lbf)

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

ChamberingP1 diameter (mm)Aexternal (cm2)Pmax (bar)Fbolt (kgf)Fbolt
5.45×39mm 10.000.78543,8002,98529,268  N (6,580  lbf )
.223 Remington 9.580.72084,3003,09930,396 N (6,833 lbf)
7.62×39mm 11.351.01183,5503,59235,223 N (7,918 lbf)
6mm ARC 11.200.98523,8003,74436,714 N (8,254 lbf)
.303 British 11.681.07153,6503,91138,352 N (8,622 lbf)
7.92×57mm Mauser 11.971.11973,9004,36742,824 N (9,627 lbf)
7.65×53mm Mauser / 7×57mm 12.011.13293,9004,41843,327 N (9,740 lbf)
6.5×55mm 12.201.16903,8004,44243,563 N (9,793 lbf)
.30-06 Springfield / .308 Winchester 11.961.12344,1504,66245,722 N (10,279 lbf)
7.62×54mmR 12.371.20183,9004,68745,964 N (10,333 lbf)
8mm Lebel 13.771.48923,2004,76546,734 N (10,506 lbf)
7.5×55mm Swiss GP 11 12.641.25483,8004,76846,761 N (10,512 lbf)
.375 Holland & Holland Magnum / .300 Winchester Magnum 13.031.33354,3005,73456,230 N (12,640 lbf)
6.5×68mm / 8×68mm S 13.301.38934,4006,11359,947 N (13,477 lbf)
.375 Ruger / .416 Ruger 13.521.43564,3006,17360,539 N (13,610 lbf)
.277 FURY (SAAMI specifications)11.951.12165,5166,18760,670 N (13,640 lbf)
.300 Remington Ultra Magnum 13.971.53284,4006,74466,139 N (14,869 lbf)
.300 Winchester Short Magnum 14.121.56594,4006,89067,567 N (15,190 lbf)
.338 Lapua Magnum 14.911.74604,2007,33371,914 N (16,167 lbf)
.300 Norma Magnum / .338 Norma Magnum 14.871.73664,4007,64174,935 N (16,846 lbf)
.300 Lapua Magnum / 7.62 UKM 14.911.74604,4007,80776,556 N (17,210 lbf)
.375 CheyTac / .408 CheyTac 16.182.05614,4008,22480,654 N (18,132 lbf)
.50 BMG 20.423.27493,70012,117118,829 N (26,714 lbf)
14.5×114mm 26.955.70443,60020,536201,387 N (45,274 lbf)

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

See also

References