While the bullet is in the air, there are no horizontal forces acting on it (we're neglecting air resistance and there is gravity, but that is a vertical that no affect x-movement). Therefore, the acceleration of the bullet in the x-direction can be solved as a rectilinear motion problem.
we have the next information
v=100 m/s
t=1 s
We will use the next formula
where v is the velocity, d is the distance and t is the time
we substitute the values given
then we isolate the d
After 1 second, the bullet will be 100 m away.