From "The horse's mouth" as it were!http://www.ruger.com/service/FAQs.html#Q60
Can I dry fire my Ruger P-Series and .22 pistols?
Yes. All Ruger pistols can be dry fired without damage, and dry firing can be useful to familiarize the owner with the firearm. However, be sure any firearm is completely unloaded before dry firing!
As I understand it, there is a firing-pin stop on the MK .22's that prevents the pin hitting the chamber face. Woe to the person that does not re-install it correctly I guess... I, however, am not an expert gunsmith, nor sadly, do I portray one on TV. But I have dry-snapped my MKIII as have other folks without ill effect.