Tracers Rounds (From SimHQ)

[Defender]

LASER:

Light Amplification by Stimulated Emission of Radiation

The tracers in COD look nothing like lasers, lasers are focused continual beams of light, not individual bolts of energy.

Tracer rounds in WWII planes are meant as an aiming aid, so they're quite obviously visible from directly behind so that the gunner can see where his shots are going. Of course it the streaks lengthen as you move out to the x axis and they're moving so fast the eye behaves like a camera shudder creating glorified streaks.