Revealing mistake: When Jim is sitting in the back of the police car, after taking the two policemen hostage, John shoots at the two cops. When the cop on Jim's right gets shot and the window smashes, blood jets up from behind him - not from his neck. (00:44:30)
Continuity mistake: When Jim eats at Nash's workplace, in one shot there is no napkin, knife or fork in front of Jim's plate. Then when Nash walks away it cuts to another angle, and now there is a knife and fork on a napkin. (00:28:00)
Continuity mistake: When Jim stops for coffee in "Roy's Cafe," John sits opposite him, and if you watch closely in one of the shots, the plant behind Jim gets knocked over, yet a few shots later it's back upright standing in its container. (00:49:15)
Continuity mistake: When C. Thomas Howell is looking in the mirror, in the shots taken from above the mirror, it is not that dirty. Yet, when it cuts to a close up of the mirror, it is filthy.
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.