That would be a key part in determine the position of Zimmerman when the shot was fired. If it is upward, then Zimmerman must have been on the ground with Martin on top.
If horizontal, then most likely both were standing, and perhaps an unwarranted shooting.
What’s interesting about the gun is a full magazine was recovered indicating a new round didn’t cycle after being fired. This suggests a struggle from both for control of the gun or a weak grip when fired, preventing the slide from operating.
I think you are asking about the trajectory of the bullet wrt the ground. How might that be possible to determine?