I've often thought that a stretched 9mm case - same base & rim dia as a .223 would be pretty interesting. OAL would be matched to factory ar15 mags. Size it to use .357 and .35 Rem bullets, and add a fast twist to use heavier bullets.
Seems like it would offer good punch super & subsonic for hunting & defense. It could use the same magazines and bolts as a 5.56.
