Nick Carter to AJ McLean -- Mind If I Hit You in the Nuts with a Baseball Bat?

The Backstreet Boys are still a bunch of jackasses ... just ask Nick Carter, who went Johnny Knoxville all over AJ McLean's balls today ... WITH A BASEBALL BAT ... and it was all caught on tape.

The stunt took place during a photo shoot with celeb photographer Tyler Shields. We really have no other explanation for why this happened.

Now ... here are a couple of our favorite BSB songs ... Quit Playin' Games (With My Testicles), Ball I Have to Give, and Everybody (Backstreet's Sack).