Since Enter the Gungeon does not show you the hitbox of projectiles or your character, it can be unclear at first what part of your body has to be kept clear of the bullets to avoid damage.

An example hitbox of the Pilot and a bullet.

By frame advancing a 60fps footage of the Pilot taking damage from each direction, we can determine the place and shape of the Pilot's hitbox. It starts above the pants, ends below his curl of hair and extends to either side of his head. (The example image may be off by +/- 1 pixel, pending additional testing.)

In contrast, a normal bullet's hitbox is simply its center pixels. If any of those overlap with the indicated player's hitbox, you will take damage.