[jka]

As far as I can tell, the 500k-3m figure is from an unpublished[1] report that was commissioned by the CDC - further validation could be helpful.

There is also separate disputation of the figure in a report[2] by Harvard's Injury Control Research Center.

[1] - https://www.washingtonpost.com/news/wonk/wp/2015/01/16/the-s...

[2] - https://www.hsph.harvard.edu/hicrc/firearms-research/gun-thr...