Factual error: Towards the last scene, Howell and Hauer share the same shotgun (Ryder to take pop shots at Jim in the jeep, Jim to ultimately fire three shots to kill Ryder). The shotgun used can hold at most seven or eight shells, not the 12 or more that are fired between Ryder and Jim starting from when Ryder escaped the bus.
Factual error: All the police officers are described on the cast as 'troopers', but those who arrest Jim at the diner and those who pull him off the bus are actually deputy sheriffs, not state troopers. They are not wearing uniform, are not driving cars with state police badges, are wearing stars instead of police shields, the place where he is taken after being arrested has 'Sheriff Station No.7' written on the outside wall, and the name of the sheriff's department can be seen when he is being photographed.
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.