making firearm calibers matter

Because all zombies die from one headshot, weapon calibers don't really matter much unless you're fighting a juggernaut or shooting at a plague heart or you have rotten luck and just can't seem to find anything but a certain kind of firearm. 

Here's some ways to make calibers matter:

.22: Add a percentage chance that a headshot won't kill. The zombie will stagger back, but needs another hit. 

9mm, .45: no change

.357: high chance of knockdown or dismemberment. Body shots will stun ferals.

.44: Same as .357, but also has a 40% chance to one-shot blood ferals and pop helmets. Gives a real reason to pack those vipers and vultures as a sidearm. 

Shotguns: no change.

5.56: no change. Already the optimal weapon to fight juggernauts with.

7.62: Kills blood ferals in a single headshot at close range. In addition, does an additional 25% damage to hostile humans with body shots than they do currently. It shouldn't take a full clip from an AK to kill a single unarmored human even at the highest difficulty. They're humans, not tanks.