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 materiel 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/free recoil.

Contents

Calculating bolt thrust

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

Formula

\vec{F_{bolt}} = P_{max} \cdot A_{internal}. [1]

where:

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

Area = \pi r^2 \approx 3{.}1416 \cdot r^2.

where:

Equivalently, denoting the diameter of the circle by d.

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.

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.

\vec{F_{bolt}} = P_{max} \cdot A_{external}.

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

Chambering P1 diameter (mm) Aexternal (cm2) Pmax (bar) Fbolt (kgf) Fbolt
.22 Long Rifle 5.74 0.2587 1,650 435 4,268 N (959 lbf)
9x19 mm Parabellum 9.93 0.7744 2,350 1,820 17,847 N (4,012 lbf)
.357 SIG 10.77 0.9110 3,050 2,779 27,248 N (6,126 lbf)
.380 ACP 9.70 0.7390 1,500 1,130 11,085 N (2,492 lbf)
.40 S&W 10.77 0.9110 2,250 2,050 20,101 N (4,519 lbf)
10 mm Auto 10.81 0.9178 2,300 2,111 20,701 N (4,654 lbf)
.45 ACP 12.09 1.1671 1,300 1,517 14,879 N (3,345 lbf)
.454 Casull 12.13 1.1556 3,900 4,507 44,197 N (9,936 lbf)
.500 S&W Magnum 13.46 1.4229 4,270 6,076 59,584 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

Chambering P1 diameter (mm) Aexternal (cm2) Pmax (bar) Fbolt (kgf) Fbolt
5.45x39mm 10.00 0.7854 3,800 2,985 29,268 N (6,580 lbf)
.223 Remington 9.58 0.7208 4,300 3,099 30,396 N (6,833 lbf)
7.62x39mm 11.35 1.0118 3,550 3,592 35,223 N (7,918 lbf)
.308 Winchester 11.96 1.1234 4,150 4,662 45,722 N (10,279 lbf)
.300 Winchester Magnum 13.03 1.3335 4,300 5,734 56,230 N (12,640 lbf)
.300 WSM 14.12 1.5659 4,450 6,968 68,334 N (15,362 lbf)
.338 Lapua Magnum 14.91 1.7460 4,200 7,333 71,914 N (16,167 lbf)
.300 Lapua Magnum 14.91 1.7460 4,700 8,339 81,776 N (18,384 lbf)
.50 BMG 20.42 3.2749 3,700 12,117 118,829 N (26,714 lbf)
14.5x114mm 26.95 5.7044 3,600 20,536 201,387 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.

References

  1. ^ A Look at Bolt Lug Strength By Dan Lilja
  2. ^ Stolle Panda Bolt Stress and Deflection Analysis