Halo 4 was rated M. Halo 5: Guardians is rated "T for teen." What's going on here? An M rating is the video game industry's equivalent of a film being rated R, i.e. adult players only.

Rated M, for mature gamers age 17 and older, these titles. combined to sell a whopping 9.3 million copies, even though they didn't hit. stores until the last two months of the year.

In that first study, retailers sold M-rated games to children 85 percent of the time. At the time, only music with explicit lyrics was as easy for children to purchase, according to the FTC.

Most video games rated "M" for mature audiences fail to disclose violent content on their labels and can easily fall into the hands of children, according to a study released on Monday.