The study comes from the Annenberg Public Policy Center at the University of Pennsylvania. It looked at nearly 1,000 movies, the 30 top-grossing films from each year dating back to 1950.
And while the authors say that by definition a PG-13 movie should have less violence than an R-rated one, they've found the complete opposite.
"A top-grossing R-rated film of today will have less gun violence than a PG-13 top-grossing film," said Daniel Romer, director of Annenberg's Adolescent Communication Institute.
Romer was one of the people who worked on the new study, which concludes that the amount of gun violence in PG-13 movies has more than tripled since 1985.
The reason? Romer says violence that used to bump a film into the "R" category has now been deemed by the Motion Picture Association of America (MPAA) to be acceptable for children to see.
"The 'Die Hard' series used to be rated R, now the latest version of that is PG-13," said Romer.
The same goes for movies like the "Terminator" series, which Romer says are just as violent these days, and are now accessible to younger audiences. That, in turn, industry experts point out, means a bigger audience and a potentially bigger payoff.
"You can have a blockbuster that is R-rated, but they're few and far between," said Matthew Belloni, executive editor of The Hollywood Reporter.
Belloni covers the film industry for The Hollywood Reporter. He says studios lobby hard to get their movies the much more profitable PG-13 rating from the MPAA.
"The ratings board allows them to stretch the limit of what is acceptable for a PG-13 movie," said Belloni.
A spokesperson for the MPAA refused to comment on the gun violence study.
The Annenberg organization is hoping the study will convince the industry to move films with a lot of gun violence back into the "R" category, which Romer says seems to be reserved for movies with sexual content.
"The movie industry uses that to restrict young children because they worry that if they see sex on the screen, they may start wanting to have sex," said Romer.
The question posed by the new study: Shouldn't that fear apply to kids watching violence?
Some people say the amount of violence in PG-13 movies is worrisome.
"Hollywood reacts to one thing, and that's money," said Belloni. "If these films stop doing well at the box office, then Hollywood will change. Otherwise it's going to be more of the same."
The study also found that 94 percent of the films they analyzed had at least one five-minute-long segment of gun violence.
The Annenberg study is set to be published in next month's edition of Pediatrics.