A few years back then ATK came into our range with a few hundred pounds of ballistic gel and a lot of ammo to do a demonstration. The cliff notes were, they can produce almost any caliber that will penetrate and expand to a specification. They at that time told us they were capable of providing 9mm ammo that would equal the current 40 cal specifications and at a much reduced cost. The proved it with their gel tests. Since that time many agencies have been going back to the nine for its benefits of lower cost, shoot-ability, and increased capacity in a sidearm.
Interesting side note was their testing of 380 which also has come a long way in performance.