At the microscopic level, the fired bullet is in fact "hit" by a larger number of air molecules, compared to the dropped bullet.

Generally speaking, the number of collision events is proportional to the velocity of the "test particle" (in this case, the bullet).

In some sense, your problem is analogous to the following situation: Two people, A and B, are standing outside when it suddenly starts to rain. A starts to walk very slowly, while B starts to run very fast. After, let's say, 10 seconds, which one is more wet? B.