The statistics may not be as clear cut as your question..
Example, I'm sure some firearms purchased legally are not intended for misuse, such as a crime of passion- husband or wife catching their beloved with another.
My guess would be only a small portion of ppl who commit violent gun crimes purchase the weapon with said crime in mind, I do not doubt it happens but my guess is most are committed with stolen or borrowed guns.
I could be way off base buts that my gut reaction to your question. Hence the lack if available statistic..
Someone once said 75.6% percent of statistics are made up anyways.. Go figure..