OMFG!!! COMPUTER SIMULATED AI DOESN'T ACT HUMAN!!!
I have yet to see a game that does AI half as good as HL2. I did the exactly the same thing in farcry, I crouched in a bush and threw rocks (you can throw rocks to make sound and destract opponents if you haven't played the game yet...) at a snipers head for almost a full 15 minutes (I was reacearching weather rocks acctually did damage when thrown at opponents, they don't) Most 3D games have areas where the AI's detection is horribly weak, the only way to mask the overlooked areas is to make the game simple enough that there is nothing to hide behind and exploit it. Most 3D games would simply rather risk AI irregularities being exploited then build the game in such a way they that they can't. HL AI has always worked based on point detection, the enemies don't shoot at you unless thier ocular unit has a clear line of sight at some predetermined point on your body (in HL2's case it seems to be the orgin where your crosshair meets your model, which is sensical considering that most of the time that means if you can shoot them they can shoot you)... This is why you can hide behind walls and put full clips of LMG into offence chambers if you sneak up just right; offence chambers won't attack you unless they can see the center of your body.
If you can find a game that those conditions aren't true for I will give you a cookie and then some.
I don't know anything about the way the hitboxes are managed in HL2, but if you can improve an onos hitbox to the point you can put shots between its legs, you can do the same, then you can improve the combine hitboxes too. Better valve be lazy on one condition like hitboxes then practically every game model the way ID did with doom 3.
Newsflash: HL2 doesn't use pixle hit detection! or did you miss that info when it came out long long ago.
[edit] sorry, I didn't mean to sound offense tooky, I just get frustrated with the level of realistic simulation that people expect out of the HL2 engine. I mean, for God sakes, it's a game engine! If you want real life get off your damn computer!