I disagree Challenge
The bullet is falling at its terminal velocity as soon as it leaves the gun. Falling for that long, the drag is going to eventually stop its forward motion, or nearly so. Leaving you with only its downward terminal velocity.
I think a .50 is going to lose half its muzzle velocity in the first second, or 800 yards, beyond that it will be slowing rapidly.
Falling for over 30 seconds gives the bullet lots of time to shed minor forward movement via drag.
Interesting exercise!
In the vertical direction it is accelerating downward as soon as it leaves the gun, but terminal velocity is not achieved instantaneously. Gravitational acceleration is 32 ft/s^2. Terminal velocity would be when it accelerates to a speed (vertical) where the vertical component of drag is equal to the gravitational force.
Acceleration is how fast something is speeding up or slowing down. For the case of drag, when velocity is high so is drag, as velocity decreases so does the rate at which it is decreasing. What this means is that the bullet is slowing down the fastest when it first leaves the muzzle, so out at 800 yards it is actually slowing down slower than when it first starts off.
In the end I think it would still have several hundred ft/s horizontal velocity left in it when it got to the ground.