Jim Halsey: Do I look like a killer to you?
Proprietor: Well, what happened to you?
Jim Halsey: Nothing.
Proprietor: Nothing?
Jim Halsey: Nothing happened.
John Ryder: You wanna know what happens to an eyeball when it gets punctured? Do you got any idea how much blood jets out of a guy's neck when his throat's been slit?
Jim Halsey: What do you want?
John Ryder: I want you to stop me.
Jim Halsey: Why are you doing this to me?
John Ryder: You're a smart kid... figure it out.
Answer: What you see him playing with is a link on the chains around his waist - note chains, not handcuffs. We know he's a resourceful guy, and just before he escapes, we see the two guards playing cards, with the shotgun loosely on one's lap - based on what we know about the configuration of the truck, John would have easily been able to leap forward and grab the shotgun while we see Jim driving up to the truck. John then shoots the guards with the shotgun we now know he just grabbed.