There are quite a few 9mm rounds available, that's true. But a .40 calibre or the next smaller round, a 10mm...... Are more likely to put a bad guy down with fewer rounds. My youngest son works for a PD here in Texas and the decision to go from .40 calibre Glocks to 9mm was made by the bean counters. Not because the smaller 9mm round has better ballistics.
Following the changeover to 9mm, his PD had to revise tactics and training. Now they must fire quite a few more rounds to put a bad guy down and neutralize the immediate threat, so they are taught to move in closer whilst continuing to fire until the bad guy is down. That's why you hear the anti-gunners freaking out about PD firing so many rounds at a bad guy. They see it as overreacting. But its really because it takes more 9mm rounds to put them down now.
