OK the more times I read this the more I think about what I do. The more I think about what I do the more I have to disagree.

"In concept, a piercing gun is similar to a staple or nail gun. It uses force to push an object (the earring stud) into a surface (the ear). Most ear piercing guns are made from plastic and metals, and can be purchased without any special license or required training from beauty supply stores and manufacturers. Their use is widespread, and almost every mainstream ear piercing facility (Claire's, Merle Norman, Wal-Mart and beauty salon type places) uses them.

The way they work seems fairly simple: with an earring stud in place in the adapter, and a backing in the cradle, the tension regulator is pulled back or "****ed", the point of the stud is used as a guide for aim and the trigger is pulled, forcing the stud through the flesh. As of yet there are no widespread certifications or government regulated training requirements associated with the use of piercing guns. Doing this procedure is about as complicated as stapling a few sheets of paper together.
Risks and dangers of getting pierced with a gun :
Excessive swelling and infection is almost a guarantee with this type of piercing. It may seem simple and harmless, but the procedure is actually very unsanitary and risky.

1) Sanitation because of the plastic componenents and cost of proper sanitizing equipment, piercing guns are not properly sterilized. They are not cleaned using any method that is widely used in medicine or tattooing. Antiseptic wipes are not able to kill germs instantly, and do not usually kill blood borne pathogens like Hepatitis or HIV. Many salespeople are told to do this in front of the customer to reassure them and gain their trust, while some never do it at all.

2) Blunt force trauma because they need to be worn for a long period of time, piercing studs are not very sharp, many have rounded tips, with grooves in the post so that the backing fastens securely to the stud. Essentially, these blunt points with grooves and notches shred their way through healthy tissue using the force and speed the gun provides. Because a stud is not streamlined it increases the amount of damage to the tissues and causes excesive swelling.

3) Malfunction piercing guns malfunction frequently and in many different ways. Often the earring adapter will not release the earring once it is already in place, inside the ear, and it has to be removed by pulling at it with pliers. Often times the piercing guns will not pierce all the way through a customer's ear, leaving a stud pushed part ways through the lobe, and a puzzled salesperson. When that happens options are to try and manually push the earring the rest of the way through, or remove it and pierce it again. Both are extremely painful.

4) Studs because of the way a piercing gun works, it is not possible to be pierced using anything other than stud earrings, this causes a problem in many people who experience excessive swelling, because as the ear swells, the backing and front of the stud pinch it, which causes more swelling, and eventually infection. Stud earrings also do not allow for as thorough cleaning as do hoop earrings, and while some people remove the studs early and change to hoops, this only adds to the trauma of the tissues, and prolongs the healing process.

5) Training the training is usually minimal, often covered by reading a booklet and practicing by piercing circular cosmetic sponges. Crooked piercings are more common with poorly trained people.

6) Infants and small children run the risk of being cut in the face. The square corners and metal edges of the cradles that hold both the adapter and backing, can easily pinch the chubby cheeks of infants as the guns are held paralell to their face."
All of that was written by someone who used to work in a chain store and did this as part of her job.

Here is a short list (highly abridged) of reasons that they are
bad:

It is not possible to sterilize them. When you are pierced blood
plasma sprays out the back and that is never cleaned, nor is it possible to clean it because the gun is made of plastic and can't be put in an
autoclave.
The jewelry is usually gold-plated metal, which is not acceptable.
Gold isn't an acceptable metal grade in the piercing industry because it
is so soft and to strengthen it, nickel must be added. Nickel is a
common allergen.
The design of the jewelry also isn't appropriate for a healing
piercing. It doesn't allow room for the piercing to swell and many people have problems with the skin swelling out over top of the backing. And the backing itself is a problem. Butterfly backs aren't easily cleaned and harbour massive amounts of bacteria. True infections are extremely rare in piercings, but not rare in gun piercings.
Also, since the ear is pierced with the stud, a dull object (coming to
a point doesn't make it sharp) is violently jammed through tender
tissue. The internal scar tissue that results from gun piercings is
incredible. A lot of trauma is caused to the tissue that simply does not
happen when it's pierced with a needle.
It also hurts a looot more to be pierced with a gun than a needle,
since a needle is gentle and surgical sharp. A needle is also single use,
because once it has been used there is no way to sharpen it again.
Also, the aftercare that they give out at places (like Claire's)
that use guns is totally terrifying to anyone in the piercing industry.
The 'ear care solution' should never ever ever be used on any piercing.
It usually has alcohol in it, which is cytotoxic, so it will kill the
healing tissue as well as any germs. And if it doesn't have alcohol
then there's another compound that is a UN class 8 corrosive. Yum.
Then there's the issue that the person who's putting a hole through the skin has training that consisted of 'point and shoot' rather than an
apprenticeship of a year or more.
And that's just lobes.
If cartilage (including the nose) is pierced with a gun there is a
whole nother list of problems in addition to these.