This article is from the Piercing FAQ, by Anne Greenblatt with numerous contributions by others.
The piercing gun or piercing implement was originally intended to be
used on earlobes only. Even so, ear piercing studs are usually of one
length and too short to accommodate swelling or earlobes which are
thicker than average.
Ear piercing studs are very difficult to clean thoroughly. The
butterfly clip backing can become clogged with hair, discharge, dirt,
and bacteria. Studs also have a tendency to become wrapped with hair,
embedding the hair in the piercing.
The piercing gun procedure causes more tissue trauma than the piercing
needle procedure. The studs are considerably duller than a piercing
needle and literally tear a hole through the tissue, whereas the
needle slices a clean hole the gauge of the jewelry to be installed. A
cleanly made hole is especially important in cartilage piercings.
Most importantly, the cleanliness of the piercing gun method is
questioned. Ear piercing guns are often made of plastic which cannot
be properly sterilized in an autoclave. Simply wiping the gun with a
surface disinfectant between clients is not adequate when the piercing
gun could have possibly been exposed to bloodborne pathogens. Although
the piercing studs themselves may be sterile, piercing guns which
touch the earlobe before and after the procedure can be easily
contaminated with blood. The gun may also be indirectly contaminated
if the piercer touches the studs after they are installed and then
touches the piercing gun without changing gloves.
While some newer piercing guns are designed with sterilized,
encapsulated stud cartridges to reduce the risk of contamination of
the gun, older models without this safeguard are still used and are
readily available for sale.
From The Centers for Disease Control and Prevention
"In 1985, CDC issued routine precautions that all personal service
workers, such as hairdressers, barbers, cosmetologists, and massage
therapists should follow, even though there is no evidence of
transmission from a personal service worker to a client or vice
versa. Instruments that are intended to go through or penetrate the
skin, such as tattooing and acupuncture needles or ear piercing
devices, should be used once and thrown away or thoroughly cleaned
and then sterilized. Instruments such as razors, which are not
intended to penetrate the skin but which may become contaminated
with blood should be used for only one client and thrown away or
thoroughly cleaned and disinfected after each use."