In two previous studies on violence against law enforcement officers conducted by the authors, offenders stated their reason for selecting a particular firearm as availability, 41 per cent in the first study and 68 percent in the second.6 These offenders did not care about bullet weight or velocity. The majority of the offenders in both studies had been involved in prior shootings before assaulting or killing the officers. Their major concern was being “fast on the trigger” and delivering the bullet to its intended target. One stated, “There’s no time to sight up the gun. If you hesitate, you’re dead.”
Another thing I found interesting is that far more police officers have been killed with 9mm than any other caliber (the 2nd most "cop-killing" caliber was .38 special). I think this is mainly due to the availability of 9mm and .38 special handguns, though another factor to consider is that both of those handguns typically have low recoil (allowing faster and more accurate follow up shots) and 9mm handguns usually have very high capacities.
Finally, did you notice that half of all cops killed were wearing body armor??? That leads me to believe that a lot of cops were killed by headshots.