>>64428294
You're describing 9x25mm Dillon. I think 7.5 FK's "thing" is the higher pressure, around 50,000 PSI CIP, which is probably why its case is so thick. It's like a bigger and hotter .22 TCM. I think the best way about it is a shortened .30 Rem/6.8 SPC case for around 22-25 gr H2O capacity and necked down to .224 with an Ackley or .224 Boz-style shoulder, and 5.56x24mm-style neck dimensions for projectile commonality. With a fast powder around 50kpsi it should be pretty awesome. You could get some decently heavy projectiles in there too if you make it long enough.