Barnes could have grabbed a handgun off the helicopter pilot, but he wanted to suck in a cong with that nice bit of shiny loot on a pilot with a grenade wedged in between him and his chair. When Charlie jostles the body, he gets a bonus frag along with the handgun.

Barnes is a shifty fucker that way. 

Edit: Fixed the post, since Tearstone had his guy rummage the cockpit first.