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.
Revealing mistake: When Jim hits the brakes in his stolen patrol car, the two pursuing police cars turn and flip over and over. As they roll, the 'roll cannon' is visible on the underside of one of the cars (a round steel tube in a hole in the floor) and at left, the two cannon slugs from both cars bounce down the road. This device fires the slug downward, and the force behind it lifts the car when the slug hits the road, launching the car over.
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.