In ancient times, when I was in basic training in the German army, we learned the safety distance for a G3 (7.52x51) was 3000m, the effective combat range of the same gun when mounted was 900m.
I'm guessing your training involved projectiles travelling in a mostly horizontal direction. In a vertical shot, gravity is acting directly against the direction of travel, which means the downward acceleration of gravity is 100% against the direction of the forward momentum.