Piercing with a gun is a very bad idea, for several reasons.
As mentioned, gun piercing can shatter cartilage.
The studs are not sharp enough, so it's forcing something blunt through your ear, which creates far more damage to the surrounding tissue than a sharp needle does -leading to longer a healing process.
The studs are a set size, meaning they are sometimes not long enough and can get embedded in the ear lobe or not allow room for any swelling.
Piercing guns are not sterile, no matter what people try to tell you! Maybe the gun part doesn't actually touch your ear, but it is near enough to your ear to get contaminated by 'back-spray' and cannot be autoclaved. That gun has been near so many other people's ears. Do you really want it near yours?
The studs are often poor quality metals or metals that are not best suited to healing a piercing or to preventing allergic reactions, eg: gold is more porous that titanium so more likely to harbour bacteria; silver tarnishes, the coatings come off plated jewellery... The recommended metal is implant grade titanium, or at a push implant grade stainless steel. In the UK, the standard is titanium.
Also, the people who pierce in places such as Claire's usually don't have training in bloodbourne pathogens and how to avoid cross contamination.
Here are some links:
BME Encyclopaedia
and again
Daelner, on BME
Wikipedia
I can't find the other link i was going to post... will edit if i find it.
I'm getting an error message on the BME site
i think it's been taken down.
Hilly - I'm really glad you complained. I'd encourage anyone else who has also had a bad experience to complain too. Maybe then stores such as Claire's will realise that their procedure is just not acceptable. At the very least they might be inspired to clean up their act -literally.