Not so much.
http://www.washingtonpost.com/blogs/fact-checker/post/the-stale-claim-that-40-percent-of-gun-sales-lack-background-checks/2013/01/20/e42ec050-629a-11e2-b05a-605528f6b712_blog.html
http://www.nationalreview.com/articles/338735/40-percent-myth-john-lott#