A tattoo needle should reach down into the dermis layer of the skin, which means that on the tattoo gun, one must hold down the armature band and adjust the depth of the needle to somewhere between 1 and 2 millimeters. If the needle is too shallow, then the ink eventually comes out of the skin, but if the needle is too deep, it causes pain and damages the skin.
Skin is made up of three different layers. The outside layer is called the epidermis, the middle layer is called the dermis and the deepest layer is called the subcutaneous tissue.
The skin of the epidermis layer is constantly flaking off and being pushed out, which means if the ink from a tattoo gun only went into the epidermis, the tattoo would eventually flake away. For this reason, tattoo needles need to penetrate into the dermis layer of the skin, which is usually reached at a depth between 1 and 2 millimeters.
All tattoo guns are operated differently, but typically, the needles come with specific compatible tips. The depth of the needle can be adjusted by holding down the armature band, which places the needle in its lowest position, and then turning the contact screw until the needle is between 1 and 2 millimeters off of the tip of the gun.