Not every crime is committed by a defined criminal, many are committed by those committing a crime for the first time. Now that is so easy due to the accesibility of the tools. Firearms being so readily available will cause regular citizens to occasionally use them to harm others, it doesn't mean they have been obtained illegally.

Every other 1st world country has proven prohibition does mean a significantly decrease risk of being shot. That is in per 100,000 people, therefore number of citizens does not matter.