If you shoot it parallel to the bottom of the canyon it will fall at 32 feet/ sec. squared . If you shoot it toward the bottom it will travel at muzzle velocity to the bottom .
At 1500 fps you cannot ignore drag. If you shoot straight down, air resistance initially will certainly be a lot greater than gravity. When drag equals gravity, you are at "terminal velocity".
To a first order, for a car traveling at 40 MPH drag is a lot less than gravity.
I took physics 1.01 with Professor Resnick and he had a famous lecture demonstation, with a rifle pointed at a stuffed toy monkey suspended about the stage by an electro-magnet. He would throw a switch and the current to the magnet would be cut at the same instant that the rifle was discharged. The monkey would be hit after falling about three feet, even though the rifle was initially aimed directly at him. The monkey and bullet underwent the same vertical acceleration.