
Continuity mistake: In one scene C. Thomas Howell almost runs head-on with an 18 wheeler, but just misses it. However, the truck clips the car he is driving and clearly takes the bumper clean off. The next driving scene shows the bumper back on the car.
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.