Which is the more powerful handgun, the 9mm, the 375 sig, or the 40 S&W? Why is the 9mm used so much by law enforcement and military?

The 40 S&W is hands down the most powerful cartridge out of the 9mm, the 357 sig and the 40 S&W. The 9mm flirts with 400 ft-lbs of energy but never really gets there. It propels bullets weighing from 100 grains to 125 grains to do it. The 357 sig hits the middle of 400 ft-lbs easily (almost 500) with loads using 90 to 147 grain bullets. The 40S&W hits the mid 400 range with 135 to 200 grain bullets. It even gets up into the 500 ft-lbs of energy range with a few loads. (Check out Chapters 3 and 4 of the handgun data tables, it blew me away.)

I assume that the 9mm is used because; it is so common, it is a NATO round, and you can get a few extra rounds per magazine. But if you look at the 45 ACP, you can only get between 7 and 12 round in most of platforms that handle that cartridge. So I guess I don't have a good answer to your second question...... Money and politics most likely.

yeah I agree with the previous for the most part. Nato doesn't really mean it is the best or most efficient. It is mostly politics, what is in place and what the manufacturers are building the different weapons around. It is hard to be the new guy in the military unless the military asks for a new guy. Then you never know what the H&*! is going to happen.

Look at the the new 6.8 SPC. Why didn't they just go with the existing 6mmx47 or just neck up the 5.56x45 up to the 6mm and create a 6mmx45? Or go with the 6.5 grendal? They both perform better than the 6.8 SPC, already existed and didn't require the research dollars. But who knows what they were really looking to accomplish. All I know is a 6mmx45 would still allow 30 rounds in the exact same mags, provide an 85 or 90 grain bullet at 2800 ftps and would be very effective out to 400 or even 500 yards. So NATO mostly means AVAILABLE IN SURPLUS AMMO. lol! Nothing else of real value. It's not like the good old 30-06 or 308 that originated from the military.