In your add for the 7.5 mmFK Brno cartridges (good price!), one comment puzzles me. You say “this caliber is not a necked down version of any other caliber.” How did you determine that?
I ask only because I have two prototypes of the 7.5 FK, formed and loaded in the Czech Republic using Starline 10 mm Magnum cases. I also have a properly headstamped, production 7.5 FK cartridge.
The prototypes I have arbitrarily designated “Prototype 1” and “Prototype 2” which are not designations assigned by the company that made them, rather simply for clarity here. Prototype 1 has a pretty normal-looking GM HP Bullet, with six splits at the mouth; Prototype 2 has a very pointed brass-color bullet with a small diameter meplat. The serial production round has a copper semi-pointed bullet, the only load I have seen to date and the same one in your ad.
Following are case measurements of the three:
Prototype 1 :
Head (rim): 1.64 mm (0.419")
Base, just above extractor-groove bevel: 10.69 mm (0.421")
Case just below the beginning of shoulder: 10.66mm (0.4195")
Prototype 2 (note, measurements are in the same order as for Prototype 1, as will be the measurements for the factory serial-production cartridge:
Head (rim): 10.66 mm (0.420")
Base: 10.69 mm (0.421")
Case below shoulder: 10.68 mm (0.4205")
It seems to me, since the factory cartridge is well within the specification spread for both of the prototypes, that the parent case for the 7.5 FK round is the 10 mm Magnum cartridge case. Brno made have made some internal modifications to the web for added strength, but the external measurements, in my mind, show the Brno cartridge to be a necked-down 10 mm Magnum.
I base my belief here on the fact that many cartridges, such as 9 mm Para, have varying internal measurements, despending on the maker, to accomodate higher pressures than envisioned with the original design of the cartridge, but we still do acknowledge the origin of the cases, and not give the cartridges a different name.
Just wondered on your comment in the ad. I am certainly not any kind of expert on case design, or metallurgy involved in redesigning a cartridge case (ie: necking down,composition of the brass material, etc.), but I feel the parentage of the 7.5 FK case is obvious.
Edited for removal of typos only.