A shot from most D20 weapons will drop a 'normal' human with a D6 HD and no CON bonus. A 5th lvl Tough Hero with an 18 Con, though, is going to take a few shells.
DR 5 is pretty serious. A .45 caliber handgun - which is a pretty hefty shell - only deals 2d6 damage. That's an average score of 7. DR 5 means that about 45% of your shots from a .45 will have *no* effect on a Zombie. I know they're the walking dead and all, but my Colts will blow big pieces off of people with each shell.
Go down to something like a .22 (2d4) and you're talking no real damage at all against DR5. Short of a crit, which conventional D&D zombies don't take.
What the expanded crit range does is this: 25% of all shots - and a higher percentage of all hits - deal 6d6 damage instead of 2d6 damage from the aforementioned Colt. That's an average of 21 points instead of 7. That should kill most 'normal' humans very dead - if they have 4 HP and go to -10, that's an instant-kill. That might be a tad high. Maybe 18-20? that's 15% of all bullet wounds pretty much immediately lethal.
It depends on a lot on the 'feel' you want for your game. If guns are meant to be effective versus people and zombies, then higher crit ranges would help that feeling - but getting shot will probably either kill or disable a PC ... much like getting shot in the real world. If you want 'hard to stop' zombies, just give them a big HD or high CON - that would make their massive damage threshold higher, FORT saves higher, and more HP in general.
- "Don't assume you are safe because there is a rational explanation for all of this."