So I was curious and started reading a little.
The 7.5 FK is 27mm in length and the base is 10.8 mm.
https://www.fkbrno.com/images/Sliders/FK-7.5-SYSTEM/THE-CARTRIDGE/DESCRIPTION/1200%20(1).jpgThe site just had general stats and I didn't see where it mentioned which bullet the numbers are for, but...: 2000 fps, 880 ft/lbs of muzzle energy.
https://www.fkbrno.com/cartridge-7-5For reference, the 10mm auto has a length of 25 mm and a base of 10.8 mm. I'm sure if I read more they have thickened case walls or something. Further, the 9x25 Dillon is putting 800 ft/lbs of muzzle energy with all the Underwood loads. It seems like a 10mm auto magnum case that is shortened and necked, at least to start the patty for the cartridge development.
https://en.m.wikipedia.org/wiki/10mm_AutoDon't get me wrong, I'm really excited for the tech inside the pistol and to see how effective it is. Especially since it seems like the H9 is exploring a similar type of system.