Chamber pressure

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Within firearms, chamber pressure is the pressure exerted by a cartridge case's outside walls on the inside of a firearm's chamber when the cartridge is fired. The SI unit for chamber pressure is the megapascal (MPa), while the American SAAMI uses the pound per square inch (psi, symbol lbf/in2) and the European CIP uses bar (1 bar is equal to 0.1 MPa).

Contents

Regardless of pressure unit used, the measuring procedure varies between CIP method, SAAMI method, and NATO EPVAT.[ clarification needed ] The chamber pressures are measured to different standards thus can not be directly compared. Chamber pressures have also historically been recorded in copper units of pressure (which for example can be denoted psi CUP, bar CUP, or MPa CUP) or lead units of pressure (LUP).[ citation needed ]

Overview

When the firing pin in a firearm strikes the primer, it ignites the powder inside the case, creating an explosion that generates a large amount of pressure, often exceeding 50,000 PSI (344.7 MPa). [1] This pressure in turn pushes the bullet out of the case mouth and into the barrel. During this explosion, the brass walls of the cartridge expand and seal against the inner walls of the chamber. This expansion is what creates chamber pressure, or the amount of force applied to the inside of the chamber by the case. Maximum safe chamber pressures for commercially available cartridges are published by bodies such as SAAMI, CIP and NATO.

Measuring methods

There are 3 basic methods for measuring chamber pressure: [2]

Copper crusher method

Through the mid-1960s, the most common way of measuring pressure was drilling a hole through the chamber of the barrel and inserting a copper slug that fit flush with the chamber walls. When a cartridge is fired, it compresses the copper slug. It is then removed, measured, and compared to the original dimensions. Using the known properties of copper, the chamber pressure can then be calculated and expressed in copper units of pressure (CUP). [3] While there are now more accurate methods of measuring chamber pressure, the copper crusher method is still used for verification purposes. [2]

The piezo method

Developed in the late 1960s, the most common method of accurately measuring chamber pressure is the Piezo method. It is similar to the copper crusher method where a hole is drilled into the chamber, but rather than a copper slug a quartz crystal transducer is inserted and attached to sensitive measuring equipment. [4] This method generally yields more accurate readings than the copper crusher, and is more cost efficient due to the fact that the transducer can be reused.

Strain gauge method

The strain gauge method is the least accurate method of measuring chamber pressure using equipment, but has the advantage of being the least expensive and requires no permanent modifications to the firearm. [2] A strain gauge is attached to the barrel just forward of the chamber. Upon firing, the barrel stretches briefly, and this stretch is measured by the gauge. [5] This method is generally reserved as a way of relatively comparing different cartridge loads as the strain gauge reading is not as accurate as a copper crusher or Piezo test.

Importance in firearm maintenance

The force that is not exerted on the chamber walls is used to push the bullet down the barrel. Because the volume of the case is relatively small, the pressure closest to the chamber will be higher than at any other point in the barrel. Compared to the temperature of the powder being burned, a relatively small amount of energy and heat is transferred from the powder in the barrel to the barrel walls. Therefore, the entire process can be considered an adiabatic process, or no heat is lost during the rapid expansion of the gases. Thus, the ideal gas law can be used to express the difference in pressure as the bullet travels along the barrel: [6]

P1(V1)κ=P2(V2)κ

or

P2=P1(V1/V2)κ

where:

Looking at this thermodynamic equation, it can be seen that the amount of pressure acting upon the bullet decreases at it travels down the barrel due to the increase of the gas volume. [6] Likewise, the part of the barrel that is subject to the highest pressures is the throat, or the point closest to the chamber at which the bullet engages the rifling. Because of this, the rifling at the throat will erode faster than the rest of the barrel.

Several measures can be taken to decrease the rate of throat erosion due to pressure, some of which can be achieved by handloading.

  1. If a bullet is seated further out (i.e. closer to the rifling) it will increase the internal volume of the case. By examining the ideal gas law, PV=nRT, it can be seen that by increasing the case volume, the pressure inside the case is reduced. This in turn reduces the chamber pressure and the amount of force exerted upon the throat.
  2. If the amount of powder is decreased (using the same gunpowder), the explosion inside the case will be smaller and will result in less pressure.

Related Research Articles

Handloading, or reloading, is the practice of making firearm cartridges by assembling the individual components, rather than purchasing mass-assembled, factory-loaded ammunition.

<span class="mw-page-title-main">Rifling</span> Gunsmithing technique

In firearms, rifling is machining helical grooves into the internal (bore) surface of a gun's barrel for the purpose of exerting torque and thus imparting a spin to a projectile around its longitudinal axis during shooting to stabilize the projectile longitudinally by conservation of angular momentum, improving its aerodynamic stability and accuracy over smoothbore designs.

<span class="mw-page-title-main">Caliber</span> Internal diameter of the barrel of a gun

In guns, particularly firearms, caliber is the specified nominal internal diameter of the gun barrel bore – regardless of how or where the bore is measured and whether the finished bore matches that specification. It is measured in inches or in millimeters. In the United States it is expressed in hundredths of an inch; in the United Kingdom in thousandths; and elsewhere in millimeters. For example, a "45 caliber" firearm has a barrel diameter of roughly 0.45 inches (11 mm). Barrel diameters can also be expressed using metric dimensions. For example, a "9 mm pistol" has a barrel diameter of about 9 millimeters. Since metric and US customary units do not convert evenly at this scale, metric conversions of caliber measured in decimal inches are typically approximations of the precise specifications in non-metric units, and vice versa.

<span class="mw-page-title-main">9×19mm Parabellum</span> Pistol cartridge designed by Georg Luger

The 9×19mm Parabellum is a rimless, tapered firearms cartridge.

Internal ballistics, a subfield of ballistics, is the study of the propulsion of a projectile.

<span class="mw-page-title-main">.223 Remington</span> Firearms cartridge

The .223 Remington is a rimless, bottlenecked rifle cartridge. It was developed in 1957 by Remington Arms and Fairchild Industries for the U.S. Continental Army Command of the United States Army as part of a project to create a small-caliber, high-velocity firearm. The .223 Remington is considered one of the most popular common-use cartridges and is currently used by a wide range of semi-automatic and manual-action rifles as well as handguns.

<span class="mw-page-title-main">Headspace (firearms)</span> Insertion depth of a cartridge in a chamber

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. Bottleneck rifle cartridges headspace on their case shoulders; rimmed cartridges headspace on the forward surfaces of their case rims; belted cartridges headspace on the forward surfaces of their case belts; rimless pistol cartridges headspace on their case mouths.

In the field of firearms and airguns, obturation denotes necessary barrel blockage or fit by a deformed soft projectile. A bullet or pellet, made of soft material and often with a concave base, will flare under the heat and pressure of firing, filling the bore and engaging the barrel's rifling. The mechanism by which an undersized soft-metal projectile enlarges to fill the barrel is, for hollow-base bullets, expansion from gas pressure within the base cavity and, for solid-base bullets, "upsetting"—the combined shortening and thickening that occurs when a malleable metal object is struck forcibly at one end.

<span class="mw-page-title-main">6.5×55mm Swedish</span>

6.5 × 55 mm Swedish, also known simply as 6.5×55mm, or in its native military as 6,5 mm patron m/94, meaning "6.5 mm cartridge model 1894", is a first-generation smokeless powder rimless bottlenecked rifle cartridge. It was introduced in the 1890s, and is still one of the most common cartridges in modern rifles built for the Scandinavian market today. The cartridge was developed in a joint Norwegian and Swedish effort starting in 1891 for use in the new service rifles then under consideration by the United Kingdoms of Sweden and Norway. In 1893, the cartridge was standardized and adopted under the name 6.5×55mm to facilitate logistical cooperation between Norway and Sweden. The two nations had independent armies and consequently the normal procedure at the time was for their respective governments to use the same ammunition and then purchase small arms of their choice. Norway adopted the Krag–Jørgensen M/1894 rifle, while Sweden adopted the Mauser m/1896 rifle design that was based on a Mauser service rifle designed around the 7×57mm Mauser cartridge.

<span class="mw-page-title-main">Polygonal rifling</span>

Polygonal rifling is a type of gun barrel rifling where the traditional sharp-edged "lands and grooves" are replaced by less pronounced "hills and valleys", so the barrel bore has a polygonal cross-sectional profile.

A proof test is a form of stress test to demonstrate the fitness of a load-bearing 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 that 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.

Copper units of pressure or CUP, and the related lead units of pressure or LUP, are terms applied to pressure measurements used in the field of internal ballistics for the estimation of chamber pressures in firearms. These terms were adopted by convention to indicate that the pressure values were measured by copper crusher and lead crusher gauges respectively. In recent years, they have been replaced by the adoption of more modern piezoelectric pressure gauges that more accurately measure chamber pressures and generally give significantly higher pressure values. This nomenclature was adopted to avoid confusion and the potentially dangerous interchange of pressure values and standards made by different types of pressure gauges. For example, it makes little sense to describe a maximum pressure as 300 MPa, and in case the pressure has been measured according to the CUP procedure it should be denoted as 300 MPa (CUP).

<span class="mw-page-title-main">7mm Remington Magnum</span>

The 7mm Remington Magnum rifle cartridge was introduced as a commercially available round in 1962, along with the new Remington Model 700 bolt-action rifle. It is a member of the belted magnum family that is directly derived from the venerable .375 H&H Magnum. The original purpose of the belted magnum concept taken from the .300 H&H Magnum and .375 H&H Magnum, was to provide precise headspace control, since the sloping shoulders, while easing cartridge extraction, were unsuitable for this purpose. Improved cartridge extraction reliability is desirable while hunting dangerous game, in particular when a fast follow-up shot is required. The 7mm Remington Magnum is based on the commercial .264 Winchester Magnum, .338 Winchester Magnum, and .458 Winchester Magnum, which were based on the same belted .300 H&H Magnum and .375 H&H Magnum cases, trimmed to nearly the same length as the .270 Weatherby Magnum.

<span class="mw-page-title-main">8mm Remington Magnum</span>

The 8mm Remington Magnum belted rifle cartridge was introduced by Remington Arms Company in 1978 as a new chambering for the model 700 BDL rifle. The 8mm Remington Magnum's parent case is the .375 H&H Magnum. It is a very long and powerful cartridge that cannot be used in standard length actions, such as those that accommodate the .30-06 Springfield.

<span class="mw-page-title-main">8×68mm S</span>

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.

QuickLOAD is an internal ballistics predictor computer program for firearms.

The following are terms related to firearms and ammunition topics.

<span class="mw-page-title-main">.284 Winchester</span>

The .284 Winchester is a cartridge that has enjoyed a resurgence due to interest from long-range competitive shooters. Winchester has continued to produce brass cases for this since 1963. Introduced by Winchester in 1963, the .284 Winchester was designed to achieve .270 Winchester and .280 Remington performance from the new Winchester Model 100 autoloader and Winchester Model 88 lever-action rifles.

Small arms ammunition pressure testing is used to establish standards for maximum average peak pressures of chamberings, as well as determining the safety of particular loads for the purposes of new load development. In metallic cartridges, peak pressure can vary based on propellant used, primers used, charge weight, projectile type, projectile seating depth, neck tension, chamber throat/lead parameters. In shotshells, the primary factors are charge weight, projectile weight, wad type, hull construction, and crimp quality.

<span class="mw-page-title-main">Freebore</span>

In firearms, freebore is the portion of the gun barrel between the chamber and the rifled section of the barrel bore. The freebore is located just forward of the chamber neck and is cylindrical in shape. The diameter of the freebore is larger than the groove diameter of the gun barrel bore so that no rifling is present and projectiles used in the firearm can accelerate through the freebore without resistance.

References

  1. ↑ "Archived copy" (PDF). Archived from the original (PDF) on 2013-05-11. Retrieved 2013-04-15.{{cite web}}: CS1 maint: archived copy as title (link)
  2. 1 2 3 "Metallic Cartridge Chamber Pressure Measurement".
  3. ↑ Fundamentals of Materials Science and Engineering, Fourth Edition, John Wiley and Sons, Hoboken, 2012, p. 217
  4. ↑ Fundamentals of Materials Science and Engineering, Fourth Edition, John Wiley and Sons, Hoboken, 2012, p. 527
  5. ↑ Mechanics of Materials: An Integrated Learning System, John Wiley and Sons, Hoboken, 2011, p. 547
  6. 1 2 Fundamentals of Engineering Thermodynamics, Seventh Edition, John Wiley and Sons, Hoboken, 2011, p. 49
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