Power factor (shooting sports)

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A high-speed photograph of a .38 Special bullet fired out of a Smith & Wesson Model 686 revolver. Bullet coming from S&W.jpg
A high-speed photograph of a .38 Special bullet fired out of a Smith & Wesson Model 686 revolver.

Power factor (PF) in practical shooting competitions refers to a ranking system used to reward cartridges with more recoil. Power factor is a measure of the momentum of the bullet (scaled product of the bullet's mass and velocity), which to some degree reflects the recoil impulse from the firearm onto the shooter (see section on limitations).

Contents

Power factor is used in competitions sanctioned by the International Practical Shooting Confederation (IPSC), United States Practical Shooting Association (USPSA), Bianchi Cup, Steel Challenge and International Defensive Pistol Association (IDPA).

Calculation

The PF is based on the bullet's momentum as it is moving through the air by measuring the bullet speed using a chronograph and measuring a similar bullet mass on a weighing scale, thereafter calculating the power factor by the formula:

Units

The PF can be represented using different units:

Alternative English Engineering units are slug foot per second (slug⋅ft/s) or pound force seconds (lbf⋅s).

Conversion from imperial to metric

Conversion from kilo grain feet per second to newton-seconds is trivial, since both the grain and foot is based on SI units. One troy grain is defined as 0.06479891  grams exactly [1] and one international foot is defined as 0.3048  meters , giving the exact conversion factor of 0.019750707768. [2]

For example:

Conversion from metric to imperial

Conversion from newton-seconds to kilo grain feet per second can lead to an added rounding error due to the conversion numbers having repeating decimals. The conversion factor from meters to feet, 10.3048, has a period of 42, [3] and the conversion factor from grains to grams, 10.06479891, has a period of 24288. [4] The complete conversion factor is thus 10.019750707768 ≈ 50.63109695846926066472495437713875449407641703911215502117... (repeating period of 170016. [5] )

For example:

Examples of different loads

The table below are examples, and for the same caliber different bullet weights can be used. Bullet velocity depends, along with other factors, on bullet weight, powder types used and barrel length for the particular firearm. Some cartridges not suitable for competition are included for reference.

CartridgeBullet mass
(grains)
Muzzle velocity
(ft/s)
Power factor
(kgr·ft/s)
Bullet mass
(gram)
Muzzle velocity
(m/s)
Power factor
(newton seconds)
.22 Long Rifle (5.6×15mmR)40 gr1080 ft/s43 kgr·ft/s2.60 g330 m/s0.86 N⋅s
.30 Carbine (7.62×33mm)110 gr1920 ft/s211 kgr·ft/s7.10 g585 m/s4.16 N⋅s
.380 ACP (9×17mm)95 gr955 ft/s91 kgr·ft/s6.20 g291 m/s1.80 N⋅s
9×18mm Makarov 93 gr978 ft/s91 kgr·ft/s6.00 g298 m/s1.79 N⋅s
9×19mm 116 gr1150 ft/s133 kgr·ft/s7.50 g350 m/s2.63 N⋅s
.38 Special (9×29mmR)158 gr950 ft/s150 kgr·ft/s10.20 g290 m/s2.96 N⋅s
9×19mm +P124 gr1250 ft/s155 kgr·ft/s8.00 g381 m/s3.05 N⋅s
.38 Special +P (9×29mmR)158 gr1000 ft/s158 kgr·ft/s10.20 g300 m/s3.06 N⋅s
.38 Special +P+ (9×29mmR)158 gr1150 ft/s183 kgr·ft/s10.20 g351 m/s3.58 N⋅s
.38 Super (9×23mmSR)130 gr1280 ft/s166 kgr·ft/s8.40 g390 m/s3.28 N⋅s
.357 SIG (9×22mm)125 gr1430 ft/s179 kgr·ft/s8.10 g436 m/s3.53 N⋅s
.357 Magnum (9×33mmR)158 gr1450 ft/s229 kgr·ft/s10.20 g440 m/s4.49 N⋅s
.40 S&W (10×22mm)155 gr1200 ft/s186 kgr·ft/s10.20 g366 m/s3.73 N⋅s
10mm Auto (10×25mm)220 gr1250 ft/s275 kgr·ft/s14.30 g381 m/s5.45 N⋅s
.41 Remington Magnum +P+ (10.4×33mmR)265 gr1490 ft/s395 kgr·ft/s17.20 g454 m/s7.81 N⋅s
.44 Remington Magnum +P+ (10.9×33mmR)340 gr1470 ft/s500 kgr·ft/s22.10 g448 m/s9.90 N⋅s
.45 ACP (11.43×23mm)230 gr850 ft/s195 kgr·ft/s14.90 g260 m/s3.87 N⋅s
5.45×39mm 56 gr2887 ft/s162 kgr·ft/s3.60 g880 m/s3.17 N⋅s
5.56×45mm NATO 62 gr3100 ft/s192 kgr·ft/s4.00 g945 m/s3.78 N⋅s
6.5mm Creedmoor (6.5×48mm)129 gr2940 ft/s379 kgr·ft/s8.40 g896 m/s7.53 N⋅s
.300 AAC Blackout (7.62×35mm)125 gr2215 ft/s277 kgr·ft/s8.00 g675 m/s5.40 N⋅s
7.62×39mm 122 gr2356 ft/s287 kgr·ft/s7.90 g718 m/s5.67 N⋅s
7.62×51mm NATO 175 gr2590 ft/s453 kgr·ft/s11.00 g790 m/s8.69 N⋅s
.308 Winchester (7.62×51mm)175 gr2645 ft/s463 kgr·ft/s11.00 g810 m/s8.91 N⋅s
.30-06 (7.62×63mm)180 gr2900 ft/s522 kgr·ft/s11.70 g884 m/s10.34 N⋅s
12 gauge-70 mm 492 gr (118 oz)1200 ft/s591 kgr·ft/s32.00 g366 m/s11.71 N⋅s
12.7×99mm NATO (.50 BMG)750 gr2820 ft/s2115 kgr·ft/s50.00 g860 m/s43.00 N⋅s
.950 JDJ (24.1×70mm)2400 gr2100 ft/s5040 kgr·ft/s160.00 g640 m/s102.40 N⋅s

Verifying during competition

For all major practical shooting competitions, claimed power factors are checked by firing the competitor's gun and ammunition through a chronograph. The most common practice is for a competitor to provide a certain number of rounds to the Range/ Safety Officers at the beginning of the match day. During the course of the match, the Range/Safety Officers at the chronograph station will pull a bullet from the competitor's ammunition and weigh it. When the competitor arrives at the chronograph station, they provide their pistol and an empty magazine to the Range/Safety Officer who loads a number of rounds into the magazine and fires them through the chronograph to determine the ammunition's velocity. The power factor is verified against the competitor's claimed power factor. If a competitor claims major power factor and fails to achieve it, they are moved to minor and their targets are scored accordingly. If a competitor fails to make minor power factor, they generally can continue to shoot the match, but will do so for no score.

At local (often referred to as "club") matches, it is rare to verify the competitors' claimed power factor, except to ensure the minimum caliber is met (e.g., an IPSC Standard shooter using a 9 mm bullet and claiming Major power factor).

Limitations

Measuring the power factor is a quick and easy way to measure recoil, but has some drawbacks. While the power factor measures the momentum of the fired bullet, it doesn't take into consideration the firearm weight or the contribution from the ejected propellant gases which together makes up for the total recoil impulse from the firearm. The contribution from propellant gases depends on the amount of gunpowder and the effectiveness of any muzzle brake or suppressor. The power factor also doesn't take into consideration any angular momentum or torque, i.e. any lever arm caused by an offset between the bore axis of the firearm to the contact point on the shooter. Also, recoil can alternatively be measured by recoil velocity or recoil energy instead of by recoil impulse.

An alternative formula for simplified recoil measurement which also takes into account firearm weight, gun powder weight and the velocity of the gun powder gases has been suggested: [6]

Gunpowder velocity depends on the amount and type of powder (burn rate) etc. Assuming that the average velocity of the powder gases is half the bullet muzzle velocity, the formula can be simplified to the following:

Various shooting organizations

International Practical Shooting Confederation

IPSC provides the two power factors minor and major, which have different scoring points on paper targets. The IPSC target has the three scoring zones; A, C, and D, with the points 5-3-1 for minor and 5-4-2 for major. Good hits are scored equally regardless of power factor, while lesser hits are penalized less with major power factor. The concept historically originates from the idea that a lesser hit with a harder hitting caliber will be more likely to end a firefight in a real life situation. However, modern ballistic studies have shown no noticeable difference in terminal performance correlating to the recoil difference between typical minor and major handgun calibers. [7] On the other hand, the power factor requirement has been kept to award the greater challenge of marksmanship required by competitors to shoot firearms with more recoil well.

In IPSC competitions all handguns must have a power factor of at least 125 kgr·ft/s (≈ 2.47 newton seconds), and therefore almost anything of 9×19mm or greater caliber will meet the minimum required power factor. The minimum power factor rule is designed to mitigate the speed and accuracy advantages of smaller calibers. Less-powerful cartridges have less recoil, and therefore can be fired more quickly with the same accuracy. Setting a minimum power factor value requires recoil management skills by all competitors.

DivisionMinor
scoring
Major
scoring
Handgun
Production
125 kgr·ft/s (≈ 2.47 N⋅s)
Only minor scoring
Handgun
Standard, classic, revolver
125 kgr·ft/s (≈ 2.47 N⋅s)170 kgr·ft/s (≈ 3.36 N⋅s)
Handgun
Open
125 kgr·ft/s (≈ 2.47 N⋅s)160 kgr·ft/s (≈ 3.16 N⋅s)
Rifle
All divisions
150 kgr·ft/s (≈ 2.96 N⋅s)320 kgr·ft/s (≈ 6.32 N⋅s)
Shotgun
All divisions
480 kgr·ft/s (≈ 9.48 N⋅s)
Only major scoring

Handgun

The desire to get the benefit of major scoring has led to some competitors adopting high speed 9 mm cartridges loaded to major, such as .38 Super. The felt recoil of a normal handgun in .38 Super is similar to the .45 ACP, but the higher pressure in the .38 Super provides more gas pressure for muzzle brakes. The .38 Super caliber became popular in the early 1980s, and has continued to be popular in the open division after its introduction in the 1993 season.

  • In the open division, IPSC has a lower power factor requirement of 160 kgr·ft/s for major, while the other IPSC handgun divisions require a power factor of 170 kgr·ft/s for major. Open and revolver are also the only divisions that allows major scoring with a 9 mm bullet diameter (the other handgun divisions require a 10 mm bullet diameter). Together with muzzle brakes only being allowed in the Open division, this has made the .38 Super and 9×19mm cartridge loaded to major become a popular option in the open division in recent years. Higher operating pressures of the cartridges like the 9×19mm major and .38 Super are preferred over, for instance, the .45 ACP, since it provides more gases for the compensator, dampening the felt recoil and muzzle rise more. Other handgun divisions are not permitted muzzle brakes, making 9 mm major and .38 Super uncommon outside open and revolver.
  • In the standard division, the bullet caliber must be at least 10 mm in order to achieve major scoring, leading to less magazine capacity, and the competitor therefore has to make a choice between minor and major considering the difference in recoil, magazine capacity and points on target. It is a common belief that major scoring will give better results, but the main downside is more expensive and less available ammunition.
  • In the production division extra points are not awarded for Major power factor, and thus the competition is dominated by 9×19mm caliber loaded to minor power factor. This, amongst other reasons, is why the production division currently is the most popular division.
  • In the classic division a choice can be made between up to a 10-shot capacity 1911 with minor scoring (e.g. 9×19mm) or up to an eight-shot capacity 1911 with major scoring (e.g. .40 S&W or .45 ACP).
  • In the revolver division major scoring can be achieved with a 9 mm bullet diameter, but with a power factor of 170. A popular cartridge in revolver is the .45 ACP since it is easier to use with speed loaders. From 2017, revolvers with a capacity of seven rounds or more are scored with minor power factor.

Rifle

In both the open and standard rifle divisions, minor power factor is dominating due to less recoil, and 5.56×45mm/ .223 Rem is a common caliber. Ammunition loaded to major power factor such as 7.62×51 mm/ .308 Win has a bigger recoil impulse, and is regarded as having an advantage on stages with targets at long range.

Shotgun

In shotgun only a major power factor of 480 kgr·ft/s is used throughout all divisions. 12 gauge is the most common caliber, and most handle the recoil well. The extra power of a 12 gauge gives the competitors better margins of error when it comes to hit steel targets. Although 12 gauge is most prevalent for both male and female shooters, it is also possible to use other calibers, such as the smaller 16 or 20 gauge. The smallest 20 gauge has the advantage that it is easier to carry and for the shooter to keep balance while shooting, thus making it easier to shoot more accurately. However the smaller caliber requires more precision, especially on small steel targets.

United States Practical Shooting Association

USPSA also provide minor and major scoring based on power factor. Metric targets are divided into the scoring areas A, C and D with minor and major points 5-3-3-1 and 5-4-4-2 respectively, while IPSC targets are divided into the scoring areas A, C and D with minor and major points 5-3-1 and 5-4-2 respectively.

In USPSA as of 2014, a value of 165 kgr·ft/s or greater is considered major for all divisions (except production and carry optics where there only is minor scoring), while values below 165 kgr·ft/s are minor. [8] Until a point in the late 1990s, the cut off point for "making major" was 175 kgr·ft/s.

DivisionMinor
scoring
Major
scoring
Handgun
Production, carry optics
125 kgr·ft/s (≈ 2.47 N⋅s)
Only minor scoring
Handgun
Open, limited, limited-10, revolver
125 kgr·ft/s (≈ 2.47 N⋅s)165 kgr·ft/s (≈ 3.26 N⋅s)

Steel Challenge

A minimum power factor of 120 kgr·ft/s (≈ 2.37 N⋅s) was previously required to activate the stop plate which stops the competitors time, but since around 2011 the stop plates are no longer connected to timers. Instead regular shot timers that register the last shot fired are used. Therefore, the minimum power factor rule no longer applies, and competitors are encouraged to use low power factor ammo in order to preserve the steel targets.

International Defensive Pistol Association

The IDPA also has different power factors, and they are calculated the same way, however there is no scoring distinction as in USPSA. A particular IDPA division has a minimum power factor, and the firearm/ammunition combination must meet or exceed this minimum to be legal for competition. The minimum power factor was previously set as 125 kgr·ft/s for all divisions, except custom defensive pistol (which is restricted to semiautomatics chambered for .45 ACP), which had a power factor of 165 kgr·ft/s. [9]

In all instances, it is up to the competitor to ensure his or her ammunition meets the requirements for the competition or equipment division.

Effective on 17 January 2011, the power factor for the stock service revolver division (SSR) was reduced to 105 kgr·ft/s since most factory produced .38 special ammunition available on the market did not meet the previous minimum power factor of 125 kgr·ft/s. [10]

To correlate with commonly available ammunition for firearms within a given division, revised power factors were established, effective 1 January 2017. [11]

IDPA divisionPower factor
SSP (stock service pistol),

ESP (enhanced service pistol),

CCP (compact carry pistol)

125 kgr·ft/s (≈ 2.47 N⋅s)
CDP (custom defense pistol)165 kgr·ft/s (≈ 3.26 N⋅s)
Stock Rev105 kgr·ft/s (≈ 2.07 N⋅s)
Enhanced rev155 kgr·ft/s (≈ 3.06 N⋅s)
BUG (back up gun)95 kgr·ft/s (≈ 1.88 N⋅s)

NRA action pistol

In the Bianchi Cup, all matches requiring the use of centerfire ammunition must meet or exceed a power factor of 120 kgr·ft/s ( ≈ 2.37 N⋅s).

See also

Related Research Articles

<span class="mw-page-title-main">Table of handgun and rifle cartridges</span> Small arms cartridge data

This is a table of selected pistol/submachine gun and rifle/machine gun cartridges by common name. Data values are the highest found for the cartridge, and might not occur in the same load.

<span class="mw-page-title-main">.357 SIG</span> Pistol cartridge designed by SIG Sauer and Federal Premium Ammunition

The .357 SIG is a bottlenecked rimless centerfire handgun cartridge developed by the Swiss-German firearms manufacturer SIG Sauer, in cooperation with ammunition manufacturer Federal Premium. The cartridge is used by a number of law enforcement agencies.

<span class="mw-page-title-main">.38 Super</span> Pistol cartridge designed by Colts Manufacturing Company, LLC

The .38 Super, also known as .38 Super Auto, .38 Super Automatic, .38 Super Automatic +P , .38 Super +P , or 9×23mmSR, is a pistol cartridge that fires a 0.356-inch-diameter (9.04 mm) bullet. It was introduced in the late 1920s as a higher pressure loading of the .38 ACP, also known as .38 Auto. The older .38 ACP cartridge propels a 130-grain (8.4 g) bullet at 1,050 ft/s (320.0 m/s), whereas the .38 Super pushes the same bullet at 1,280 ft/s (390.1 m/s). The .38 Super has gained distinction as the caliber of choice for many top practical shooting competitors; it remains one of the dominant calibers in IPSC competition.

<span class="mw-page-title-main">.22 Long Rifle</span> Common ammunition cartridge

The .22 Long Rifle, also known as the .22LR or 5.6×15mmR, is a long-established variety of .22 caliber rimfire ammunition originating from the United States. It is used in a wide range of firearms including rifles, pistols, revolvers, and submachine guns.

<span class="mw-page-title-main">Practical shooting</span> Shooting sport based around precision, power, and speed

Practical shooting, also known as dynamic shooting or action shooting, is a set of shooting sports in which the competitors try to unite the three principles of precision, power, and speed, by using a firearm of a certain minimum power factor to score as many points as possible during the shortest time. While scoring systems vary between organizations, each measures the time in which the course is completed, with penalties for inaccurate shooting. The courses are called "stages", and are shot individually by the shooters. Usually the shooter must move and shoot from several positions, fire under or over obstacles and in other unfamiliar positions. There are no standard exercises or set arrangement of the targets, and the courses are often designed so that the shooter must be inventive, and therefore the solutions of exercises sometimes vary between shooters.

<span class="mw-page-title-main">International Practical Shooting Confederation</span> International organization for the sport of practical shooting

The International Practical Shooting Confederation (IPSC) is the world's largest shooting sport association, and the largest and oldest within practical shooting. Founded in 1976, the IPSC nowadays affiliates over 100 regions from Africa, Americas, Asia, Europe, the Middle East, and Oceania. Competitions are held with pistols, revolvers, rifles, and shotguns, and the competitors are divided into different divisions based on firearm and equipment features. While everyone in a division competes in the Overall category, there are also separate awards for the categories Lady, Super Junior, Junior, Senior, and Super Senior.

<span class="mw-page-title-main">.458 Winchester Magnum</span> Rifle cartridge

The .458 Winchester Magnum is a belted, straight-taper cased, Big Five game rifle cartridge. It was introduced commercially in 1956 by Winchester and first chambered in the Winchester Model 70 African rifle. It was designed to compete against the .450 Nitro Express and the .470 Nitro Express cartridges used in big bore British double rifles. The .458 Winchester Magnum remains one of the most popular large game cartridges, and most major ammunition manufacturers offer a selection of .458 ammunition.

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

A racegun or race gun is a firearm that has been modified for accuracy, speed, and reliability. Often a semi-automatic pistol, raceguns are used primarily in practical shooting competitions and are modified to function best within a certain set of rules, such as weight, size, and capacity requirements.

<span class="mw-page-title-main">.30-30 Winchester</span> Rifle cartridge designed by the Winchester Repeating Arms Company

The .30-30 Winchester cartridge was first marketed for the Winchester Model 1894 lever-action rifle in 1895. The .30-30, as it is most commonly known, along with the .25-35 Winchester, was offered that year as the United States' first small-bore sporting rifle cartridges designed for smokeless powder. Since its introduction, it has been utilized alongside the development of flatter shooting cartridges, most prominently those derived from designs subsidized by interest in military expenditures. The .30-30 has remained in widespread use almost entirely because of reliable effectiveness in civilian applications, and has put food on the table for millions of people in hunting situations.

<span class="mw-page-title-main">.400 Cor-Bon</span> Automatic pistol cartridge developed by Cor-Bon in 1997

The .400 Corbon (10.2x22mm) is an automatic pistol cartridge developed by Cor-Bon in 1997. It was created to mimic the ballistics of the 10 mm Auto cartridge in a .45 ACP form factor.

<span class="mw-page-title-main">Overpressure ammunition</span> Type of small arms ammunition

Overpressure ammunition, commonly designated as +P or +P+, is small arms ammunition that has been loaded to produce a higher internal pressure when fired than is standard for ammunition of its caliber, but less than the pressures generated by a proof round. This is done typically to produce ammunition with higher muzzle velocity, muzzle energy, and stopping power, such as ammunition used for security, defensive, or hunting purposes. Because of this, +P ammunition is typically found in handgun calibers which might be used for paramilitary forces, armed security, and defensive purposes.

<span class="mw-page-title-main">United States Practical Shooting Association</span> National governing organization

The United States Practical Shooting Association(USPSA) is the national governing body of practical shooting in the United States under the International Practical Shooting Confederation (IPSC). Its over 35,000 active members and over 500 affiliated clubs make USPSA the largest practical shooting organization in the United States and the second largest region within IPSC after the Russian Federation of Practical Shooting. USPSA publishes a monthly member magazine called Front Sight.

<span class="mw-page-title-main">4.5×26mm MKR</span> Swedish prototype rifle cartridge

The 4.5mm MKR, also known as the 4.5mm Interdynamic or 4.5mm Kjellgren, was a Swedish prototype rimfire cartridge developed for the prototype Interdynamics MKR bullpup assault rifle and carbine.

Free recoil / Frecoil is a vernacular term or jargon for recoil energy of a firearm not supported from behind. Free recoil denotes the translational kinetic energy (Et) imparted to the shooter of a small arm when discharged and is expressed in joules (J), or foot-pound force (ft·lbf) for non-SI units of measure. More generally, the term refers to the recoil of a free-standing firearm, in contrast to a firearm securely bolted to or braced by a massive mount or wall. Free recoil should not be confused with recoil:

<span class="mw-page-title-main">9×25mm Dillon</span> Pistol cartridge

The 9×25mm Dillon is a pistol wildcat cartridge developed for use in USPSA/IPSC Open guns. The cartridge is made by necking down a 10mm Auto case to 9 mm.

<span class="mw-page-title-main">9×23mm Winchester</span> Pistol cartridge designed by John Ricco and Anthony Valdez

The 9×23mm Winchester is a pistol cartridge developed as a joint venture by Winchester Ammunition and Colt's Manufacturing Company. The 9×23mm Winchester has a convoluted development history, but was commercially introduced by Winchester in 1996. Marketed primarily to competition shooters as a replacement for .38 Super for International Practical Shooting Confederation, United States Practical Shooting Association and International Defensive Pistol Association competition, the cartridge failed to find significant market success despite a high-profile introduction.

<span class="mw-page-title-main">Dynamic Sports Shooting Norway</span>

Dynamic Sports Shooting Norway (DSSN), NorwegianDynamisk Sportsskyting Norge (DSSN), is the Norwegian association for practical shooting under the International Practical Shooting Confederation. It was constituted in 1979 as Norges Forbund for Praktisk Skyting (NFPS), and changed its name to Dynamisk Sportsskyting Norge in 2013.

<span class="mw-page-title-main">Multigun</span> Type of practical shooting event

Multigun, Multi Gun or Multi-Gun, often also called 2-Gun or 3-Gun depending on the types of firearms used, are practical shooting events where each of the stages require the competitor to use a combination of handguns, rifles, and/or shotguns Multigun has a lot in common with ordinary IPSC/ USPSA single gun matches, and matches generally have courses of fire where the shooter must move through different stages and engage targets in a variety of different positions.

<span class="mw-page-title-main">Ashley Rheuark</span> American sport shooter

Ashley Rheuark is a highly accomplished American sport shooter who took silver medal in the standard division lady category at the 2017 IPSC Handgun World Shoot, and silver medal in the open division lady category at the 2017 IPSC Rifle World Shoot. She is also USPSA National Champion, ranked as an IDPA Distinguished Master, and has proven herself as a strong competitor in multigun (3-Gun) competitions.

9mm Major, also known as 9 Major, ammunition is loaded much hotter than typical 9mm rounds, exceeding 1,550 ft/s (470 m/s) to achieve a power factor of 165. This classification benefits competition shooters as it allows maximum magazine capacity and scoring. However, it is not readily available and dangerous to use in pistols not designed for such high pressure. Most handgun manufacturers caution against using +P and +P+ ammunition and 9mm Major loads, which generate even higher chamber pressures. In tactical situations, the added velocity may improve vest penetration, but it may hinder hollow-point projectiles' effectiveness due to over-expansion. Proper cartridge design is crucial for optimal performance.

References

  1. National Institute of Standards and Technology (October 2011). Butcher, Tina; Cook, Steve; Crown, Linda et al. eds. "Appendix C – General Tables of Units of Measurement" (PDF). Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices. NIST Handbook. 44 (2012 ed.). Washington, D.C.: U.S. Department of Commerce, Technology Administration, National Institute of Standards and Technology. ISSN 0271-4027. OCLC OCLC   58927093. Retrieved 30 June 2012.
  2. Online Calculation by Wolfram|Alpha | 0.06479891*0.3048
  3. Online Calculation by Wolfram|Alpha | 1/0.3048
  4. Online Calculation by Wolfram|Alpha | 1/0.06479891
  5. Online Calculation by Wolfram|Alpha | 1/(0.06479891*0.3048)
  6. Karl's Soapbox - Recoil Calculator
  7. FBI Training Division Justifies 9mm Caliber Selection - The Firearm Blog
  8. "USPSA Rule Book, Appendix D1" (PDF). USPSA. Retrieved 24 June 2014.
  9. "IDPA Rule Book, Appendix One" (PDF). IDPA. Retrieved 7 April 2014.
  10. "IDPA web notice". IDPA. Archived from the original on 4 February 2012. Retrieved 31 January 2012.
  11. IDPA 2017 Rulebook IDPA, Retrieved 31 Aug 2017