I may rile a few feathers here but I have seen many shot with a 9mm and the local Department got away from the 9mm when it was proven in the field to be ineffective. As an expert in court, I have also testified as to what went wrong in shootings as well as gang style use of weapons.
Any gun will kill or injure. The word kill becomes more pronounced as the choice of caliber becomes larger.
What made the 9mm popular in the US was the large ammo capacity and it's ability to fire many rounds. This was very important to those that did not practice good marksmanship. Officers in the streets began to feel outgunned when carrying a revolver with six rounds and facing a shooter with 19 rounds. Various departments began using primarily Glocks (of which I carry but not a fan of) in 9mm. Today the majority of those departments have changed to the .40 caliber for several reasons, primarily the stopping power (more on this shortly). It is more desirable to stop aggression early on. There is not much safety in a long continuing gun battle. Hitting one in the stomach with a .40 or such is more of an attention getter than with a 9mm.
But now for a couple real time examples.
Shortly after the issuance of the 9mm in place of the S&W .357 (the finest gun made in my opinion for law enforcement) there were some evident problems. Though officer involved shootings were not common beforehand or afterward, those that occurred were seeing more rounds fired. Officers had began the spray and pray type firing rather than depending on accuracy since the sidearm held more ammo. Yet the more noticible factor was the added hits it was taking to stop aggression.
One of the first departmental shootings after the introduction of the 9mm into service involved 6 officers doing a drug stop. The dealer was known to be armed and had shot others before. He was in his vehicle and was unable to flee so he began shooting. The perp was struck 16 times and lived to stand trial. Fortunately he did not injure any officers. A total of 62 rounds were fired from officers weapons. Many did not penetrate the car door or windshield.
Another shooting followed days later and an officer was critically wounded. The perp was approached in an alley behind a store. There was no place for concealment or cover and the perp began shooting a .38 special. The officer was hit twice and went down but continued firing. Later the perp sought treatment at a local hospital with a trauma center and was taken into custody after being treated for five bullet wounds. He remained mobile after being struck in the torso with five hits. Had this happened with larger caliber ammo, he would likely not have survived. The officer was struck in lesser vital areas but was in the hospital for weeks while the perp was in a cell three days later.
As was stated, the object is not to kill but cease the forward aggression of the combatant. A 9mm is not going to do that unless accuracy is at it's best. We all know accuracy is often forgotten during stressful conditions.
Since the issuance of heavier calibers in law enforcement work, the amount of hits to stop aggression has dropped.
Now as to the gangbangers. They do not practice and that is why they use the AK-47 and other weapons of large capacity. Their accuracy sucks and they need and require more guns and more rounds. Also they do not hang around to remain in gun fights. They mostly shoot by drive by since they stand a smaller chance of being struck by return fire.
Years ago, many gangs required someone shoot a victim as a show of courage as well as loyalty. They soon found that shooters were generally apprehended shortly after a shooting and the shooters were being tried as felons. Gangs learned that shooting livestock was misdemeanor offenses and handled more often with fines as opposed to jail time. Liveshock shootings went up and drive by shootings dropped some.
Sorry for this but I will put in a funny shooting as well. A group of three gangbangers were in a van with sliding doors on both sides of the vehicle and these doors were left open as they drove around looking for their intended target. The target was spotted walking down a city street and the driver handed the brother in the rear of the van a AK-47 with the instructions to shoot the individual. A major probelm here was this punk had never shot a gun in his life, yet he took the gun and proceeded to shoot out the right side of the van. Unfortunately for the others, the target began running across the road in front of the van as he attempted to make it to a large drainage ditch. He had not been hit by the novice shooter and the front seat passengers kept yelling for him to shoot. He did so following the movements of the target running in front of the van. The shooter took out both front seat gang members, hitting them at point blank range in the back of their heads and side of the face. Both died of injuries sustained. The target gets away and the shooter leaves the scene after the van jumped a curb and struck a utility pole.
Sorry for the long post but wanted to speak from experiences found in the field but to sum it up, which would you rather be hit with, a 9mm, a .40, or one of the magnums, assuming a non vital area of the abdomen. Enough said?