Halo's engine won't support "realistic" gameplay. ever. You can make realistic weapons and have them animate realistically, but the fact that you can run around and have just as much accuracy as you will crouched is kind of a throw off of the gameplay. The limitations of using tags just won't allow it. I could see the engine being used to make an arena-style shooter, but not so much something like Call of Duty. Though I have to admit Ambush is pretty fun, it's not exactly what I'd call realistic in terms of gameplay.
Besides, this is just my opinion, but why have realism when you could have crazy-ass scifi shit blasting the crap out of people? Railguns, chainguns, hand-held rocket launchers may not be realistic, but they sure are fun.
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