An answer to the OP's problem actually dawned on me a little while ago...
5.7x28
It's about the same capacity as a Hornet, but has the advantage of being a more modern shape with sharper shoulder. And being a military/LE cartridge, there is plenty of surplus brass around:
surplus 5.7x28 brass I've heard the cartridge is fussy to handload, and you have to be very careful if trying to reach max loads. But light loads are the topic of this thread...
Bullberry lists the caliber for custom Contender barrels:
Bullberry Contender barrels There are of course FN pistols & rifles, and AR uppers that run the 5.7x28, but a Contender barrel seems more versatile.
