Great physics aspects to everyone. There is another theory as well. I have a graduate degree in engineering and love questions like this to pose to my friends and colleagues. The guys in my office also believe that the expansion of HOT gasses as they leave the barrel behind, around and in front of the bullet will rise immediately and cause an 'updraft' if you will. This in turn causes a minimal rise in the initial trajectory of the round.

Just a theory and food for thought.

Thanks for a great fact and theory question.

[8D]