It’s been a while since we’ve done one of these Modern PDW Calibers installments, but we’re back, and today we’re looking at a very new round on the market, one that is currently making some pretty big waves in the pistol world. I am talking of course about the 7.5x27mm FK Brno, designed for the CZ-75-derived FK Field Pistol from the company that shares its name. A high velocity .30 cal pistol round is not a new idea, having predecessors in the .300 JAWS, 7.62×25 Tokarev, and others, but what makes the 7.5 FK so interesting is just how powerful it is: A 103 grain monolithic bullet is advertised as leaving the 6″ Field Pistol barrel at an incredible 2,000 ft/s! This means that, if the company’s performance claims are true, the FK Field Pistol is ballistically the equal of the old WWII-era M1 Carbine!
The Field Pistol itself isn’t generally considered to be a PDW, but it does share several characteristics with the PDW concept as a whole, and FK has even developed an attachable folding shoulder stock for the gun. This does raise the question of whether some kind of pistol-rifle intermediate, maybe halfway between the Field Pistol and the B+T USW, wouldn’t be a good match for a round like the 7.5mm FK.
Onto the ballistics:
One of the things that will be a recurring theme in this series on PDW calibers is how difficult it is for high velocity rounds to match the downrange energy retention of even the humble 9mm NATO. Even the monstrous 7.5mm FK, with its 71% greater muzzle energy than the 9mm, only produces slightly more energy at 300 meters, despite the ballistic coefficients of the two rounds behind very similar. Matching the downrange energy of conventional pistol rounds like the 9mm and .45 presents problems for high velocity PDW rounds because of the dramatically increased drag of the supersonic flight regime. While .45 ACP is subsonic, and the 9mm becomes subsonic not long after it leaves the muzzle, rounds like the 5.7x28mm and 7.5x27mm stay supersonic for hundreds of meters, causing them to bleed velocity much more quickly as a result. With very low (in the case of 5.7mm) or only modest (in the case of 7.5mm) projectile weights, this means that after a certain distance these rounds will have shed their velocity advantage versus the 9mm, and therefore their energy advantage as well. For the 7.5mm FK, this isn’t a big problem as 300m is really the extreme end of its effective range, but the graph still serves as an illustration of this problem. For smaller calibers like the 5.7mm, however, low energy retention at moderate ranges is a real issue, and one of the biggest challenges I have found in designing small caliber, high velocity PDW rounds!
Since I do not have a 7.5x27mm FK round in my possession, I cannot say how heavy it is, but I can take a quick guess. The 7.5x27mm FK’s case is very close to that of the 10mm Auto, and its propellant load should be similar as well. I measured a 180gr (11.7 g) 10mm Automatic load at 17 grams per round, which gives us mass for the case + propellant + primer of 5.3 grams. If we add to that the 103 gr (6.7 g) projectile of the 7.5 FK, we get a per shot mass of approximately 12 grams, which in reality possibly shakes out to a little more – I wouldn’t be surprised to find that actual specimens weighed 12.5 or even 13 grams, depending on the mass of propellant and case wall thickness.