My understanding is that many people prefer to put the Aimpoint as far down the barrel as possible, towards the front sight assembly. That way, you can more easily and more rapidly acquire the target, particularly in a CQB situation such as room-clearing. Having the optic further towards the rear, closer to the shooter, makes it harder to pick up the target and still keep both eyes open and see the whole situation going on around you. In other words, the farther away from the shooter that the Aimpoint is located, the easier it is to avoid the tunnel vision that occurs when looking through a scope, even one that is a 1X optic like the Aimpoint Comp M4.
Now, having said all of that, I will also say that I have an Aimpoint Comp M4 and I have it placed on the rails on top of the barrel, just past the point where the barrel attaches to the rest of the upper. In other words, it is not on top of the receiver, but on top of the barrel, but close in to the receiver, (mine is not far down by the front sight post). I have mine that way for two reasons:
1. I do not clear too many rooms...just does not come up in my life and line of work too often. Sometimes I have to figure out where the dog has hidden one of his toys, but I usually don't shoot the toy when I find it.
2. With my vision, which is not exactly perfect, I prefer to have the Aimpoint situated closer to me, and the current position, described above, seems to work best for me.
