What is the point of the M9?

So...what is the point of the M9? My default it is still a 2 hit headshot, just like the .22, while being more expensive then the .22 pistol. I know there are perks to increase headshot damage, but wouldn't that also make a .22 a one hit headshot?

What is the point in buying 9mm and an M9A3 from the shop, if the .22 pistol is free, and ammo is cheap, where there are 0 differences?

Am I missing something? Maybe it stuns zombies more, or knocks helmets off faster?