The differences between the EXECUTOR (player) and PRAYER (target) bullets:

Of course, none of that matters if the bullets don't hit.
The EXECUTOR has a very small hitbox, located roughly in the center of the chest, as shown below.
The hitboxes of the target bullets are smaller than they appear, as well.
Creeping up into target bullets should not be necessary, and it is generally fine to avoid them completely whenever there is room.
There are places where the player must slip through small gaps, but there should be easier ways out of those situations.