It was in ST, you break a vent covers and the grunts blast the vent to hell, then you have to fight them.
Here is another official response.
The M4 will NOT be included in the mod.
Why?
Because we said so. We are the developers. We decide what the final product will entitle. Some of you will :’( while the grown ups will move on to provide insightful ideas on other aspects of the mod.
If it’s possible, I’d like to see headcrabs on zombies have yellow blood and decals, while the zombie has red blood and decals. Or just the decals would work. I get tired of seeing headcrabs bleed red. 0_o
I second this.
It would be cool but I doubt it will happen.
I always found it weirder that the zombie bled alien blood when shot in the chest, but left a red decal.
Guess the zombie is now full of DNA and blood from the headcrab but failed to wash his clothes even after years of wanderings in the wilds.
EDIT : After a look on the PC Powerplay article, our fantasy on zombie red bool could be true. Check out p.35 the pic at the center.
So, Kane, know any ways of allowing lights to pass through bullet holes in vents?
Maybe a transparent bullet decal with a light beam attached?
wut?
It is possible to have them bleed red blood out of their bodies and yellow out of the headcrab. Just like other points I’ve made that have been ignored…
i feel your pain man 
Be glad you’re not a cockroach, at least headcrabs posts are ignored and not flamed 
.
ORLY?
Yes.
I believe that may have been an engine limit in GldSrc, and possibly lazy programming in HL2.
Headcrabs mutate the tits out of people, it only makes sense that the person wearing one would bleed rainbows
The zombies are covered in red blood so it seems only natural that they bleed red blood. I’ve seen it done in the GldSrc engine so it will very likely be possible in the Source engine.
Yup but it was the entire zombie, even if you shoot at the headcrab. In HL2 they decided to make it bleed totally yellow.
Well now you’re just kicking my while I’m down.