It just doesnt seem right that you can hit a zombie two times in the chest with a shotgun slug and still have advance at you unaffected. Sure they don't feel pain, but the physics alone behind being shot would definately cause some knockback regardless of feeling. I believe each gun should have its own knockback/slowing effect on the zombie:

Rifles: Flinch and slightly slow
Shotguns: Grunt, and stumble back
Pistols: Flinch
Axe: Grunt, stumble back
Flamethrower: nothing

Or you could shoot them in the head and have them dead (again) on the ground. This isn't a realistic mod, there is no need for realistic knockback, if you are surrounded, punch a hole through them by shooting their heads rather than pushing them out of the way with chest shots.

Why waste two slugs to the chest when you can take them out with a lucky one to the head.

As for the more serious physics of the matter, they are rotting corpses with decaying flesh and bone. A pellet, slug, or bullet is going to go right through them, ripping flesh and bone out, but not slowing the zombie noticably.

Try stopping a wave of water with a 9mm... same type of case.

He has a point, even a zombie would take the force of a shot and likely "flinch" or "stumble back" thats just a law of physics.

By the way, if you had enough 9mm bullets you might stop the wall of water.

See guys, here's the problem we create with saying "BB is not a realism mod"... WHERE DO YOU DRAW THE LINE????

How can you tell someone "Oh no we can't include a chainsaw, lawnmower, or super-atomic-ray-gun, because it just isn't realistic", when they could just comeback with "Well I thought u said it wasnt a realism mod"

WHERE DO YOU DRAW THE LINE PEOPLE????
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Anyway this is the HL 1 engine and it cannot support physics so goo smoke some

Don't know mate, if the Super-atomic ray gun is like the "Neclear Fusion Cannon" of the Red Faction series, its realistic enough.

And that is close to a tactical nuclear weapon, which is quite real, but will they add it, probally not...

Realism, practicality and play value... Many suggestions cover only one or two of the three.

High caliber pistols need to slow them down... and shotguns should knock them back. Sniper rifle shoud slow them too. If it is ever implemented, a winchester should slow them down... more than a high caliber pistol but less than a shotgun.

short awnser:

if it doesnt feel right, dont do it.

long awnser:

if it does not feel right, do not do it.

Oh it feels right alright!

And to whoever said its not possible... of course it is!!! CS and TFC did it easily!!

P.S. Immoral I knew about the chainsaw (I'm a huge fan of it) I was just simply using it as an example of the line where realism crosses fun.

As TSS said, its NOT about realism or no realism. That's bs because if it were then we wouldn't have zombies limbs falling off, or have zombies do less damage if they don't have arms, or even specific body location damage. So yes, BB is NOT based on whether something is real or not, but rather based on whether somehting just "feels" right for BB.
So immoral, you saying that "BB is not a realism mod" is *not* a valid arguement.

In conclusion:
Knockback, as I said, would just "feel" right in BB, it would be as natural as seeing a zombie loose an arm when you shoot it.

Why?:
It just gives the zombie more "presence" in the game. Shooting a zombie point blank with a shotgun and having him still advance towards you full steam almost makes it feel as if you were "invisible" to the zombie, almost like he doesn't have "spacial" presence. Including knockback would help make the zombie more everpresent in that it adds the element of present fear, making the player think:
"yes, it's not a dream, he's right infront of you... see, you just shot him and he flinched, yes this *is* really happening!" - queue fear

i will never have that real feeling, because.. well.. it isnt... its a game, yknow.

now atleast im getting the feeling of him still creeping closer, while with knockbar i'd just pop a round or 2 in the chest, give me some space to run away. now its shoot or die.

how does it give you space to run away?
I'm not talking about a 5 foot knockback here. This is a flinch + grunt + stumble. Not a 50 foot field goal.

IMO It just doesn't feel immersive when a zombie continually advances without an visible sign that you and him are in the same game.

Basically, the only confirmation that you and the zombie are actually in the same game is the death notice sprite on the HUD. Other than that, you have no way of knowing you are having an effect on him (except blood/limbs etc).

Don't be haters, its a worthwhile suggestion . The point is the knockback not if its realistic or not, the only issue is this probably isn't feasible without a lot of work. IMO there are cooler things that could be done with BB using the time that would be required to add this though.