I thought the Feds carried 10mm weapons. Less tendency to overpenetrate, greater tendency to cause more damage.
Or something. I've never fired a pistol before in my life.
Most police departments are carrying .40 S&W. The popularity of 10's is decreasing. I'm guessing the former NATO round (9mm) is what the air marshalls carry.
Join the NRA, find someone who is well seasoned in firearm safety, proficient with firearms, and ask if you could go to the range with him/her.
Air marshals tend toward the .357SIG - so says my instructor who teaches them, and recommends that round.