10mm and the ******* step child 40S&W were the Bren 10's answer to a never ask question! The original Norma 200 grain ammo had energy like God's lightning bolts, but to what purpose? Are we hunting run away school busses? The FBI adopted the 10mm in a knee jerk to the Miami shoot out and then had to go to the FBI load 155gr at about 1055fps so the average agent could control it, and that begot the 40 S&W. The early twentieth century 45 ACP seems to be an answered question! How much do I need to make bad people dead?
Ivan