>>64001707
NTA.

Such a cartridge already exists. The Belgian 7.92x24VBR. It’s a shortened 30 carbine necked up to its true straight-walled caliber of 7.92mm. Picrel: three 7.92x24’s next to a 5.7x28. The capacity of an 18 round 9mm mag would increase to 20 rounds without increasing mag length.

https://military-history.fandom.com/wiki/7.92%C3%9724mm