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7.5 FK

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7.5 FK
TypePistol
Place of originCzech Republic
Production history
DesignerFK Brno Engineering s.r.o.
Designed2014
Produced2014 - present
Specifications
Case typeRimless, bottleneck
Bullet diameter7.8 mm (0.307 in)
Land diameter7.51 mm (0.296 in)
Neck diameter8.5 mm (0.335 in)
Shoulder diameter10.8 mm (0.425 in)
Base diameter10.8 mm (0.425 in)
Rim diameter10.80 mm (0.425 in)
Rim thickness1.40 mm (0.055 in)
Case length27 mm (1.063 in)
Overall length35 mm (1.378 in)
Case capacity1.50 cm3 (23.21 gr H2O)
Rifling twist270 mm (1 in 10,63 in)
Primer typeSmall pistol magnum
Maximum pressure350.0 MPa (50,763 psi)

The 7.5 FK or the 7.5 FK BRNO (designated as the 7.5 FK by the C.I.P.[1]) is a bottlenecked rimless centerfire automatic pistol cartridge developed by the Czech firearms and ammunition manufacturer FK Brno Engineering s.r.o. It is reported that the cartridge has recently been adopted by some law enforcement agencies and militaries engaged in ongoing conflicts by their special operations units. The cartridge is known for its long-range accuracy and high incapacitation capability when compared to other automatic pistol cartridges.

History

The 7.5 BRNO was developed between 2009 and 2014, for the specific purpose of providing high capacity automatic pistols the ability to engage combatant targets at a range of between 75–150 metres (246–492 ft) while retaining more kinetic energy at that range than what a 9×19mm Parabellum cartridge can generate at the muzzle/point blank range.[2]

Due to the length of the cartridge, relatively high energy, and a pressure that the cartridge generates, no modern existing, high-capacity automatic pistol design existed which could accommodate the new caliber so FK BRNO developed a specialized platform to accommodate and use this caliber.

Initially, it was chambered in the all-steel FK BRNO Field Pistol which has a unique patented recoil attenuating system but subsequently FK BRNO developed a polymer version of that that design designated as the FK PSD. This recoil attenuating system allows the powerful 7.5 FK cartridge to be fired with high level controllability and accuracy.[3]

Cartridge dimensions

The 7.5 FK has 1.50 cm3 (23.21 grain of H2O) cartridge case capacity.

7.5 FK maximum C.I.P. cartridge dimensions[1]

Americans would define the shoulder angle at alpha/2 = 30 degrees. The common rifling twist for this cartridge is 270mm (1 in 10.62" inch), 6 grooves, Ø lands = 7.5 mm, Ø grooves = 7.77mm, land width = 3.75 mm, and the primer type is small pistol.

Contrary to some published information, this cartridge is not based on a necked down 10mm Auto case, and hand loaders cannot and should not attempt to make a 7.5 case from a 10mm Auto case, as the case is longer, thicker, and operates at significantly higher pressure. Therefore, necking down and stretching the 10mm Auto would create much thinner walls at the critical shoulder/neck area and can potentially be very dangerous to the user.

According to C.I.P rulings, the 7.5 FK case can handle up to 350 MPa (50,763 psi) piezo pressure. In C.I.P regulated countries every pistol cartridge combo has to be proofed at 130% of this maximum C.I.P pressure to certify for sale to consumers.

Conversions

No current pistol caliber and no current automatic pistol design other than the products of FK BRNO can be converted to the 7.5 FK cartridge due to the longer cartridge length and higher operating pressure rendering conversion impossible. However, the opposite is possible, and done by the PSD variant. A conversion barrel allows for the use of 10mm Auto and .40 S&W ammunition or a conversion of the barrel, recoil spring, and magazine can fire 9×19mm Parabellum.

Performance

The table below shows common performance parameters for several common automatic pistol calibers using kinetic energy as the main form of power comparison, using premium high-end ammunition using a 5-inch barrel.[2]

Caliber Manufacturer Load Mass Velocity fps @ 1m Velocity fps @ 90m Energy ft/lbs @ 1m Energy ft/lbs @ 90m
9mm Luger Cor-Bon JHP+p 125 gr 1250 1016 434 286
357 Sig Cor-Bon JHP 125 gr 1425 1119 564 348
40 S&W Fiocchi JHP 165 gr 1100 942 450 325
10mm Auto Hornady JHP/XTP 180 gr 1180 1004 556 403
5.7x28 FN Speer gold Dot 40 gr 1790 1322 285 155
7.5 FK FK BRNO F5 95 gr 2000 1610 845 548

The concept of using kinetic energy tables as a measuring method for incapacitating power has been used for almost a century but has been disputed for almost as long by various different authorities, scholars, and writers as it does not take into account the projectile construction and design which may affect terminal performance significantly. It also does not take into account penetration capability which is highly influenced by projectile material, design, and sectional density. Several theories have been published that claim better prediction of incapacitating power than kinetic energy such as Tylor KO index, Hatchers theory, Lethality index, Bekker knock out formula, and Hornady HITS among others.[4] FK BRNO has published their own alternative method which they claim is more efficient and is a more accurate predictor of incapacitating power (which they call Dynamic Shock Index “DSI”).[5] They have also published a video showing the 7.5 cartridge being used to hunt large boars, in which they claim that the terminal ballistics efficiency shown in the video is due to the high DSI of these loads.[6]

Dynamic Shock Index

The DSI concept takes into account meplat diameter (meplat=frontal flat area of a projectile), the sectional density of the projectile and several velocity parameters. The mass of the projectile is not taken into account directly thought it is used in order to calculate the sectional density.

The following is the formula for the Dynamic Shock Index as published by FK BRNO:

DSI = meplat diameter x impact velocity – (1500-impact velocity) x sectional density[5]

The following are examples of how some powerful versions with very high meplat diameter of popular ammunition calibers score using muzzle velocity for illustrative purposes (as opposed to impact velocity) in comparison with the 7.5 FK.[5][2]

Cartridge Manufacturer Meplat SD Velocity fps @ 1m Velocity fps @ 90m DSI @ 1m DSI @ 90m
7.5 FK F9 FK BRNO 0.236 0.160 1920 1450 72.5 52.8
44 MAG 240 gr JHP Hornady XTP 0.255 0.185 1280 1030 50 26.4
7.5 FK F5/S95 FK BRNO 0.157 0.150 2000 1610 47.1 37.9
357 Mag 125gr JHP Mag Tec 0.248 0.140 1378 1110 43.6 25
10 MM 180gr JHP Barnes Bullets 0.236 0.160 1250 1050 37.8 22.6
45ACP 185gr +P JHP Mag Tec 0.255 0.139 1148 973 30.6 15.8
9 MM 124gr +P JHP Speer Gold Dot 0.208 0.140 1262 1054 29.8 17.7
9 MM 135gr +P JHP Hornady Critical Duty 0.157 0.153 1241 1050 18.8 14.4

References

  1. ^ a b "C.I.P. TDCC datasheet 7.5 FK" (PDF). C.I.P. Retrieved 2021-03-24.{{cite web}}: CS1 maint: url-status (link)
  2. ^ a b c Forker, Bob (2013). Ammo and Ballistics 5th edition. Safari Press. ISBN 1571574026.
  3. ^ Friedman, Ed. "FK Brno PSD Multi-Caliber Pistol". Shooting Illustrated. Retrieved 2021-03-24.{{cite web}}: CS1 maint: url-status (link)
  4. ^ "Terminal Ballistics". www.rathcoombe.net. Retrieved 2021-03-25.
  5. ^ a b c "FK Brno Ammuntion Ballistics". www.fkbrno.com. Retrieved 2021-03-25.{{cite web}}: CS1 maint: url-status (link)
  6. ^ Handgun Stopping Power Vs Dynamic Shock Index ; Real Examples using Cal. 7.5 FK Brno, retrieved 2021-03-25
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