1) Relocate each player's "singularity" (the point from where all LoS calculations are made) to the camera, instead of the soles of their boots.
2) If you can see it and you're in range, you can shoot it.
3) If (2) applies and the target has the range, it can shoot back.
Any practical/theoretical reasons why this can't be done? Just seems it would get rid of some of the more, um, bizaare situations in the game...