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.
Bolt thrust is not a measure to determine the amount of recoil or free recoil.
With a basic calculation the bolt thrust produced by a particular firearms cartridge can be calculated fairly accurately.
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 (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 during NATO 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 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).
Chambering | P1 diameter (mm) | Aexternal (cm2) | Pmax (bar) | Fbolt (kgf) | Fbolt |
---|---|---|---|---|---|
.22 Long Rifle | 5.74 | 0.2587 | 1,650 | 427 | 4,268 N (959 lbf) |
FN 5.7×28 mm | 7.95 | 0.4964 | 3,450 | 1,713 | 16,794 N (3,775 lbf ) |
9×19 mm Parabellum | 9.93 | 0.7744 | 2,350 | 1,820 | 17,847 N (4,012 lbf ) |
HK 4.6×30 mm | 8.02 | 0.5051 | 4,000 | 2,021 | 19,816 N (4,455 lbf ) |
.357 Magnum | 9.63 | 0.7284 | 3,000 | 2,185 | 21,428 N (4,817 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) |
.41 Remington Magnum | 11.05 | 0.9590 | 3,000 | 2,877 | 28,213 N (6,343 lbf) |
.44 Remington Magnum | 11.61 | 1.0587 | 2,800 | 2,964 | 29,069 N (6,535 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.
Chambering | P1 diameter (mm) | Aexternal (cm2) | Pmax (bar) | Fbolt (kgf) | Fbolt |
---|---|---|---|---|---|
5.45×39mm | 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) |
7.62×39mm | 11.35 | 1.0118 | 3,550 | 3,592 | 35,223 N (7,918 lbf) |
.303 British | 11.68 | 1.0715 | 3,650 | 3,911 | 38,352 N (8,622 lbf) |
7.92×57mm Mauser | 11.97 | 1.1197 | 3,900 | 4,367 | 42,824 N (9,627 lbf) |
7.65×53mm Mauser / 7×57mm | 12.01 | 1.1329 | 3,900 | 4,418 | 43,327 N (9,740 lbf) |
6.5×55mm | 12.20 | 1.1690 | 3,800 | 4,442 | 43,563 N (9,793 lbf) |
.30-06 Springfield / .308 Winchester | 11.96 | 1.1234 | 4,150 | 4,662 | 45,722 N (10,279 lbf) |
7.62×54mmR | 12.37 | 1.2018 | 3,900 | 4,687 | 45,964 N (10,333 lbf) |
8mm Lebel | 13.77 | 1.4892 | 3,200 | 4,765 | 46,734 N (10,506 lbf) |
7.5×55mm Swiss GP 11 | 12.64 | 1.2548 | 3,800 | 4,768 | 46,761 N (10,512 lbf) |
.375 Holland & Holland Magnum / .300 Winchester Magnum | 13.03 | 1.3335 | 4,300 | 5,734 | 56,230 N (12,640 lbf) |
6.5×68mm / 8×68mm S | 13.30 | 1.3893 | 4,400 | 6,113 | 59,947 N (13,477 lbf) |
.375 Ruger / .416 Ruger | 13.52 | 1.4356 | 4,300 | 6,173 | 60,539 N (13,610 lbf) |
.277 FURY (SAAMI specifications) | 11.95 | 1.1216 | 5,516 | 6,187 | 60,670 N (13,640 lbf) |
.300 Remington Ultra Magnum | 13.97 | 1.5328 | 4,400 | 6,744 | 66,139 N (14,869 lbf) |
.300 Winchester Short Magnum | 14.12 | 1.5659 | 4,400 | 6,890 | 67,567 N (15,190 lbf) |
.338 Lapua Magnum | 14.91 | 1.7460 | 4,200 | 7,333 | 71,914 N (16,167 lbf) |
.300 Norma Magnum / .338 Norma Magnum | 14.87 | 1.7366 | 4,400 | 7,641 | 74,935 N (16,846 lbf) |
.300 Lapua Magnum / 7.62 UKM | 14.91 | 1.7460 | 4,400 | 7,807 | 76,556 N (17,210 lbf) |
.50 BMG | 20.42 | 3.2749 | 3,700 | 12,117 | 118,829 N (26,714 lbf) |
14.5×114mm | 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.
The .357 Smith & Wesson Magnum, .357 S&W Magnum, .357 Magnum, or 9×33mmR is a smokeless powder cartridge with a 0.357 in (9.07 mm) bullet diameter. It was created by Elmer Keith, Phillip B. Sharpe, and Douglas B. Wesson of firearm manufacturers Smith & Wesson and Winchester. The .357 Magnum cartridge is notable for its highly effective terminal ballistics.
The .338 Lapua Magnum is a rimless, bottlenecked, centerfire rifle cartridge. It was developed during the 1980s as a high-powered, long-range cartridge for military snipers. Due to its use in the War in Afghanistan and the Iraq War, the cartridge has become widely available.
In firearms, headspace is the distance measured from a closed chamber's breech face to the chamber feature that limits the insertion depth of a cartridge placed in it. Used as a verb by firearms designers, headspacing refers to the act of stopping deeper cartridge insertion. The exact part of the cartridge that seats against the limiting chamber feature differs among cartridge and gun designs. In general, bottleneck rifle cartridges headspace on their case shoulders; rimmed cartridges headspace on the forward surfaces of their case rims; and rimless pistol cartridges headspace on their case mouths. The case belts on belted cartridges were originally added to allow headspacing on the belt's forward surface, But in practice, this is often vestigial, and rifles chambered for belted cartridges may well headspace them on their shoulders and still be within CIP or SAAMI dimensional limits. However, belted cartridges and their corresponding chambers at their nominal CIP or SAAMI dimensions will headspace on the belt.
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The 8×60mm S is a rimless bottlenecked centerfire cartridge of German origin, dating back to the interbellum period between World War I and World War II. The bore has the same dimensions as the German 7.92×57mm Mauser service cartridge. The 8×60mm S can, due to its 83.6 mm (3.291 in) overall length, easily be chambered in standard sized Mauser 98 bolt-action rifles. In such military M98 bolt actions internal magazine boxes feature a magazine length of 84 mm (3.307 in).
A proof test is a form of stress test to demonstrate the fitness of a load-bearing or impact-experiencing structure. An individual proof test may apply only to the unit tested, or to its design in general for mass-produced items. Such a structure is often subjected to loads above those expected in actual use, demonstrating safety and design margin. Proof testing is nominally a nondestructive test, particularly if both design margins and test levels are well-chosen. However, unit failures are by definition considered to have been destroyed for their originally-intended use and load levels.
The 7.5×54mm French, 7.5 French, or 7.5 MAS is a rimless bottlenecked rifle cartridge. It was developed by France as an update to the 7.5×57mm MAS mod. 1924 cartridge. It replaced the obsolete 8×50mmR Lebel round used during World War I, and served as the French service cartridge until superseded by the 5.56×45mm NATO and 7.62×51mm NATO cartridges in the 1970s and 1980s.
The 8×68mm S rebated rim bottlenecked centerfire rifle cartridge was developed in the 1930s by August Schüler of the August Schüler Waffenfabrik, Suhl, Germany as a magnum hunting cartridge that would just fit and function in standard-sized Mauser 98 bolt-action rifles. The bore has the same lands and grooves diameters as the German 7.92×57mm Mauser service cartridge. This is one of the early examples where a completely new rifle cartridge was developed by a gunsmith to fit a specific popular and widespread type of rifle.
The 6.5×47mm Lapua is a smokeless powder rimless bottlenecked rifle cartridge that was developed specifically for 300–1,000 m (328–1,094 yd) competition shooting by ammunition maker Nammo Lapua and the Swiss rifle manufacturer Grünig & Elmiger AG in 2005. Other common names for this cartridge include 6.5×47mm.
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The delta L problem refers to certain firearm chambers and the incompatibility of some ammunition made for that chamber. ΔL is a Commission Internationale Permanente (C.I.P.) geometric dimensioning and tolerancing definition for cartridge cases that are longer than the chamber they have to fit in. "Delta L" essentially means "difference in length".
The following are terms related to firearms and ammunition topics.
The 8×64mm S is a rimless bottlenecked centerfire cartridge developed as a military service round for the German Army who never issued it. As is customary in European cartridges the 8 denotes the 8 mm bullet caliber and the 64 denotes the 64 mm (2.52 in) case length.
The 7.62mm UKM [7.62×57mm] is a specialized rimless bottlenecked centerfire cartridge developed for long-range rifles. The commercially successful .338 Lapua Magnum cartridge has functioned as the parent case for the 7.62mm UKM, which is essentially a necked-down shortened version of the .338 Lapua Magnum. The .338 cartridge case was used for this since it has the capability to operate with high chamber pressures which, combined with smaller and hence lighter bullets result in high muzzle velocities.
Fluting is the removal of material from a cylindrical surface in a firearm, usually creating grooves. This is most often the barrel of a rifle, though it may also refer to the cylinder of a revolver or the bolt of a bolt action rifle. In contrast to rifle barrels and revolver cylinders, rifle bolts are normally helically fluted, though helical fluting is sometimes also applied to rifle barrels.
The Patrone 88 or M/88 is a rimless bottlenecked rifle cartridge. It was a first-generation smokeless propellant cartridge designed by the German Gewehr-Prüfungskommission (G.P.K.) as the then new smokeless propellant introduced as Poudre B in the 1886 pattern 8×50mmR Lebel started a military rifle ammunition revolution.
Bearing pressure is a particular case of contact mechanics often occurring in cases where a convex surface contacts a concave surface. Excessive contact pressure can lead to a typical bearing failure such as a plastic deformation similar to peening. This problem is also referred to as bearing resistance.
The Heym SR 30 is a straight-pull rifle produced in Germany by Heym. The rifle is also available in a left hand version.