7.5 FK

7.5 FK
Type	Pistol
Place of origin	Czech Republic
Production history
Designer	FK Brno Engineering s.r.o.
Designed	2014
Produced	2014 - present
Specifications
Case type	Rimless, bottleneck
Bullet diameter	7.8 mm (0.307 in)
Land diameter	7.51 mm (0.296 in)
Neck diameter	8.5 mm (0.335 in)
Shoulder diameter	10.8 mm (0.425 in)
Base diameter	10.8 mm (0.425 in)
Rim diameter	10.80 mm (0.425 in)
Rim thickness	1.40 mm (0.055 in)
Case length	27 mm (1.063 in)
Overall length	35 mm (1.378 in)
Case capacity	1.50 cm3 (23.21 gr H2O)
Rifling twist	270 mm (1 in 10,63 in)
Primer type	Small pistol magnum
Maximum pressure	350.0 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.) 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.

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 that can 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.

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

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.

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. 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”). 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.

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

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.

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