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)
Ballistic performance
Bullet mass/type Velocity Energy 6.16 g (95 gr) FKB S95 JHP 2,000 ft/s (610 m/s) 850 ft⋅lbf (1,150 J) 6.54 g (101 gr) FKB CFN 1,950 ft/s (590 m/s) 850 ft⋅lbf (1,150 J)
Test barrel length: 152mm (6")

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.

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 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 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.^[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 with the exception of the 5.7 x 28 FN caliber, though that caliber has a significantly narrower cartridge case, which renders conversion not possible.

Characteristics[]

The goal of the 7.5 FK project was to create a cartridge with a level of performance that could exceed that of the highly effective 125 grain (8.1 g) JHP 357 magnum load and at least as effective terminally as the 7.62 x 39 Russian/5.56 NATO cartridges when fired from short barrel (8-10 inches, 20- 25.2 cm) AK/AR type platforms, and to provide it in a package small enough to fit a standard-size military automatic pistol platform. Another goal was to have a flat trajectory and to be accurate enough to engage targets at 75-150 meters which is the most common engagement distance in modern combat situations and therefore to be considered as a second weapon as opposed to a secondary weapon which is what most military pistols using standard NATO and other automatic pistol calibers are considered.

Measurements of standard 7.5 cartridges loaded with 95 grain (6.16 g) showed a muzzle velocity of 2000fps (610m/s) out of a 5.3-inch barrel equal to 4-inch of free bore (13.46 cm equal to 10 cm free bore) and a muzzle energy of 845ft ft/lbs (1146 jouls)^[4] which is almost 50% higher than the venerable 357 magnum 125 grain JHP when using premium quality high pressure ammunition where the measured muzzle velocity is 1450 fps (442 m/s) and muzzle energy of 584 ft/lbs (799 jouls)^[2] when fired from a 4-inch barrel, so essentially from the same barrel length as the 7.5 FK.

More importantly the 7.5 FK cartridge achieves this performance without compromising the sectional density of the projectile which is critical for the penetrating depth and incapacitating capability. The 95 grain 7.5 FK cartridge has a sectional density of 0.150 while the 101 grain 7.5 FK cartridge has a sectional density of 0.160. In comparison the 357 magnum 125 cartridge has a sectional density of 0.140. The higher sectional density allows the 7.5 FK projectile to retain higher velocity and lower trajectory at longer ranges, while having significantly deeper penetration^[5] especially since it has 40% more velocity. It is interesting to note that the measured velocity and energy of the 95 grain load 7.5 FK cartridge at the minimum design engagement distance of 75 meters is 1650 fps (503 m/s) and 577 ft/lbs (783 joules) respectively. Which is essentially almost the same as what the 125 grain JHP 357 magnum generates at the muzzle / point black range from the same barrel length.

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]

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 influence 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.^[6] FK BRNO has published their own alternative method which they claim is more efficient and more accurate predictor of incapacitating power which they call Dynamic Shock Index (DSI).^[7] They have also published a video showing the 7.5 cartridge being used to hunt large boars where they claim that the terminal ballistics efficiency shown in the video is due to the high DSI of these loads.^[8]

Available loads[]

7.5 FK bullet types^[7]

References[]