There are many things wrong with a piercing gun.  First of all, there is no way to properly sterilize the instrument in an autoclave, because the tool is mostly plastic and would melt. The part of the tool that acts as the needle isn’t really a needle meant for piercing skin. It is more of a rounded “point” that makes more of a rupture (the culprit of the “popping” noise) rather than a gentle incision made from a proper piercing needle. This means guns are more painful, and take longer to heal. There are also many areas on our bodies that a gun simply doesn’t fit into in order to pierce.