We don’t use piercing guns for several reason.
The first and most important is that they can be a source of infection transmission. Piercing guns get a certain amount of blood and other body fluids on them during a piercing. As they are generally made of plastic, they cannot be sterilized they would melt. Also, all the nooks & crannies around the mechanism tend to trap blood/fluids where they can’t physically be reached to decontaminate.
Another issue is that piercings performed with piercing guns are in no way gentle. They rely on brute force to get jewellery that is not very sharp through the skin. This causes a significant amount more trauma than piercings done with piercing sharps, especially for piercings other than earlobe.
Also, the jewellery used by piercing guns tends to be “one-size-fits-all” and doesn’t allow for different length posts for people with different size earlobes. Just as this doesn’t work well for clothing, it doesn’t work well for body jewellery. The butterfly clasp on the back of the jewellery tends to harbour dirt, debris, and bacteria. It’s also very, very rare to find ear gun studs that are made of implant-grade materials.
For all these reasons we don’t use piercing guns and feel they are never appropriate.