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Interesting. According to the FBI, for handguns it doesn't matter the size of the caliber because to do serious damage to the tissue the projectile would have to travel 2,000 feet per second, and that velocity is only found in rifle calibers. For handguns, magazine capacity, accuracy, and recoil are far more important than the sheer size of the bullet, as it's been documented that law enforcement officers miss 7 out of 10 shots in real shootings.
So it looks like the FBI is going back to 9mm after almost 30 years of .40 S&W, just with better bullet selection, as 9mm has seen some advances in bullet design in recent years. The G19 found in RE3R is the exact handgun the FBI uses, so perhaps that's where Capcom got their inspiration from.
That's probably why we don't see too many rifle variants in RE games, because one shot kills with high capacity magazines would be cheese especially on zombies. Also, that is why zombies are able to keep stumbling at you after several shots fired into them with a 9mm. A) Zombies don't feel pain (either psychological or physical B) Zombie tissue structure has been mutated to withstand more punishment and therefore, less chance of bleeding out after to hitting vital organs.
After reading all of that, I'm amazed it's as easy as it is to take down zombies as it is in RE with a 9mm bullet, and this gives me a more scientific perspective on ballistics.