Assuming no air resistance isn't a very safe assumption. If you just consider gravity, the bullet would reach about 24,000 meters. . .
But I guess that in itself demonstrates that gravity plays a relatively small role in slowing a bullet fired vertically. In fact, if you assume that the bullet's velocity is zero at 1500m, and slows at a linear rate, gravity would act on it for 4 seconds instead of two, and account for the loss of 40 M/S instead of 20. Still pretty small potatoes.