if you shoot a great deal the advantage with the forty is 1) small pistol or rifle primer works and seats easy 2) less lead costs less sometimes. the Miami shootout changed things at the FBI. 60% of FBI arrests involve automobiles and penetration is important. I think it was Ayoob who did an article showing after action reports , medical examiners info, etc. the FBI people I knew kept that 10mm in the trunk and carried a smaller gun, only using the 10mm to qualify.