Bolt thrust or breech pressure is a term used in internal ballistics

Internal ballistics

Internal ballistics, a subfield of ballistics, is the study of a projectile's behavior from the time its propellant's igniter is initiated until it exits the gun barrel...

 and firearms (whether small arms or artillery) that describes the amount of rearward force exerted by the propellant

Propellant

A propellant is a material that produces pressurized gas that:* can be directed through a nozzle, thereby producing thrust ;...

 gases on the bolt

Bolt (firearm)

A bolt is a mechanical part of a firearm that blocks the rear of the chamber while the propellant burns.In manually-operated firearms, such as bolt-action, lever-action, and pump-action rifles and shotguns, the bolt is held fixed by its locking lugs during firing, forcing all the expanding gas...

 or breech

Breechblock

A breechblock is the part of the firearm action that closes the breech of a weapon at the moment of firing....

 of a firearm action

Firearm action

In firearms terminology, an action is the physical mechanism that manipulates cartridges and/or seals the breech. The term is also used to describe the method in which cartridges are loaded, locked, and extracted from the mechanism. Actions are generally categorized by the type of mechanism used...

 or breech

Breech-loading weapon

A breech-loading weapon is a firearm in which the cartridge or shell is inserted or loaded into a chamber integral to the rear portion of a barrel....

 when a projectile is fired. The applied force

Force

In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...

 has both magnitude and direction

Direction (geometry, geography)

Direction is the information contained in the relative position of one point with respect to another point without the distance information. Directions may be either relative to some indicated reference , or absolute according to some previously agreed upon frame of reference Direction is the...

, 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

Recoil

Recoil is the backward momentum of a gun when it is discharged. In technical terms, the recoil caused by the gun exactly balances the forward momentum of the projectile and exhaust gasses, according to Newton's third law...

/free recoil

Free recoil

This article is about the energy produced by small arms when discharged. For other uses, go to Recoil Free recoil is a vernacular term or jargon for recoil energy...

.

Calculating bolt thrust

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

Formula

where:

• F_bolt = the amount of bolt thrust
• P_max = the maximum (peak) chamber pressure of the firearms cartridge
• A_internal = the inside area (of the cartridge case head) that the propellant
  Propellant
  A propellant is a material that produces pressurized gas that:* can be directed through a nozzle, thereby producing thrust ;...
  
   gas pressure acts against

Cartridge case heads and chambers are generally circular

Circle

A circle is a simple shape of Euclidean geometry consisting of those points in a plane that are a given distance from a given point, the centre. The distance between any of the points and the centre is called the radius....

. 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.

By oiling proof rounds during NATO EPVAT testing

NATO EPVAT testing

NATO EPVAT testing is one of the three recognized classes of procedures used in the world to control the safety and quality of firearms ammunition....

 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

Caliper

A caliper is a device used to measure the distance between two opposing sides of an object. A caliper can be as simple as a compass with inward or outward-facing points...

 or micrometer

Micrometer

A micrometer , sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw used widely for precise measurement of small distances in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier,...

 or taken from the appropriate C.I.P. or SAAMI

Saami

Saami or SAAMI can stand for:*Sami people*Sami languages*Sporting Arms and Ammunition Manufacturers' Institute...

 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:

• 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 warm chamber that can result in cooking off

Cooking off

Cooking off refers to ammunition exploding prematurely due to heat in the surrounding environment. It can also refer to a technique used when throwing grenades to achieve a controlled, predictable explosion....

 (i.e. a thermally induced unintended firing).

Bolt thrust estimations for various pistol/revolver cartridges

Chambering	P1 diameter (mm)	Aexternal (cm2)	Pmax (bar Bar (unit) The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar... )	Fbolt (kgf Kilogram-force A kilogram-force , or kilopond , is a gravitational metric unit of force. It is equal to the magnitude of the force exerted by one kilogram of mass in a gravitational field... )	Fbolt
.22 Long Rifle .22 Long Rifle The .22 Long Rifle rimfire cartridge is a long established variety of ammunition, and in terms of units sold is still by far the most common in the world today. The cartridge is often referred to simply as .22 LR and various rifles, pistols, revolvers, and even some smoothbore shotguns have...	5.74	0.2587	1,650	435	4268 N (959.5 lbf)
9x19 mm Parabellum	9.93	0.7744	2,350	1,820	17847 N (4,012.2 lbf)
.357 SIG .357 SIG The .357 SIG pistol cartridge is the product of Swiss-German firearms manufacturer SIG-Sauer, in cooperation with the American ammunition manufacturer Federal Cartridge. While it is based on a .40 S&W case necked down to accept bullets, the .357 SIG brass is slightly longer...	10.77	0.9110	3,050	2,779	27248 N (6,125.6 lbf)
.380 ACP .380 ACP The .380 ACP pistol cartridge is a rimless, straight-walled pistol cartridge developed by firearms designer John Browning. The cartridge headspaces on the mouth of the case. It was introduced in 1908 by Colt, and has been a popular self-defense cartridge ever since...	9.70	0.7390	1,500	1,130	11085 N (2,492 lbf)
.40 S&W .40 S&W The .40 S&W is a rimless pistol cartridge developed jointly by major American firearms manufacturers Winchester and Smith & Wesson. The .40 S&W was developed from the ground up as a law enforcement cartridge designed to duplicate performance of the FBI's reduced velocity 10mm cartridge which could...	10.77	0.9110	2,250	2,050	20101 N (4,518.9 lbf)
10 mm Auto 10 mm Auto The 10mm Auto is a semi-automatic pistol cartridge developed by Jeff Cooper and introduced in 1983 for the Bren Ten pistol. It was initially produced by ammunitions manufacturer FFV Norma AB of Åmotfors, Sweden.Although it was selected by the F.B.I. for use in the field following the 1986 F.B.I...	10.81	0.9178	2,300	2,111	20701 N (4,653.8 lbf)
.45 ACP .45 ACP The .45 ACP , also known as the .45 Auto by C.I.P., is a cartridge designed by John Browning in 1904, for use in his prototype Colt semi-automatic .45 pistol and eventually the M1911 pistol adopted by the United States Army in 1911.-Design and history:The U.S...	12.09	1.1671	1,300	1,517	14879 N (3,344.9 lbf)
.454 Casull .454 Casull The .454 Casull is a firearm cartridge, developed in 1957 by Dick Casull and Jack Fulmer. It was first announced in November 1959 by Guns & Ammo magazine. The basic design was a lengthened and structurally improved .45 Colt case...	12.13	1.1556	3,900	4,507	44197 N (9,935.9 lbf)
.500 S&W Magnum .500 S&W Magnum The .500 S&W Magnum is a fifty-caliber semi-rimmed handgun cartridge developed by Cor-Bon in partnership with the Smith & Wesson "X-Gun" engineering team for use in the Smith & Wesson Model 500 X-frame revolver and introduced in February 2003 at the SHOT trade show...	13.46	1.4229	4,270	6,076	59584 N (13,395 lbf)

The P1 (cartridge case base) diameters and P_max 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 Bar (unit) The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar... )	Fbolt (kgf Kilogram-force A kilogram-force , or kilopond , is a gravitational metric unit of force. It is equal to the magnitude of the force exerted by one kilogram of mass in a gravitational field... )	Fbolt
5.45x39mm 5.45x39mm The Soviet 5.45×39mm cartridge is a rimless bottlenecked rifle cartridge. It was introduced into service in 1974 for use with the new AK-74 assault rifle. It gradually supplemented then largely replaced the 7.62x39mm round in service....	10.00	0.7854	3,800	2,985	29268 N (6,579.7 lbf)
.223 Remington .223 Remington The .223 Remington is a sporting cartridge with almost the same external dimensions as the 5.56×45mm NATO military cartridge. The name is commonly pronounced either two-two-three or two-twenty-three. It is loaded with a diameter, jacketed bullet, with weights ranging from , though the most common...	9.58	0.7208	4,300	3,099	30396 N (6,833.3 lbf)
7.62x39mm	11.35	1.0118	3,550	3,592	35223 N (7,918.4 lbf)
.308 Winchester .308 Winchester The .308 Winchester is a rifle cartridge and is the commercial cartridge upon which the military 7.62x51mm NATO centerfire cartridge is based. The .308 Winchester was introduced in 1952, two years prior to the NATO adoption of the 7.62x51mm NATO T65...	11.96	1.1234	4,150	4,662	45722 N (10,278.7 lbf)
.300 Winchester Magnum .300 Winchester Magnum The .300 Winchester Magnum is a popular, belted, bottlenecked magnum rifle cartridge that was introduced by Winchester Repeating Arms Company in 1963 as a member of the family of Winchester Magnum cartridges. The .300 Winchester Magnum is a magnum cartridge designed to fit in a standard length...	13.03	1.3335	4,300	5,734	56230 N (12,641 lbf)
.300 WSM .300 WSM .300 Winchester Short Magnum is a thirty caliber rebated rim bottlenecked centerfire short magnum cartridge that was introduced in 2001 by Winchester. The cartridge overall length is 72.64mm, cartridge case is 53.34mm in length and the bullet diameter is .308 in , which is common to all U.S. .30...	14.12	1.5659	4,450	6,968	68334 N (15,362.1 lbf)
.338 Lapua Magnum	14.91	1.7460	4,200	7,333	71914 N (16,166.9 lbf)
.300 Lapua Magnum .300 Lapua Magnum The .300 Lapua Magnum 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 .300 Lapua Magnum, which is essentially a necked-down version of the .338 Lapua Magnum...	14.91	1.7460	4,700	8,339	81776 N (18,384 lbf)
.50 BMG .50 BMG The .50 Browning Machine Gun or 12.7×99mm NATO is a cartridge developed for the Browning .50 caliber machine gun in the late 1910s. Entering service officially in 1921, the round is based on a greatly scaled-up .30-06 cartridge...	20.42	3.2749	3,700	12,117	118829 N (26,713.8 lbf)
14.5x114mm	26.95	5.7044	3,600	20,536	201387 N (45,273.6 lbf)

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