Sue - I don't think you're getting the joke. He's writing a note saying that he's carrying a rifle and will be back to the vehicle any time now --- just in case anyone thinks about messing around with the vehicle.

My note might say "Decided to test out the new shotgun in the clearing just down the trail. If you don't hear any shots fired that means I'm on my way back to the vehicle. ... signed Your buddy from cell block A."