Plot hole: After Blondie has "teamed up" with Angel Eyes, he hears a gunshot and says something like "Every gun sings its own tune", indicating that he's recognized the shot as being from Tuco's gun. But how did Tuco get his gun back? It certainly would have been confiscated when he entered the prison camp, and the chance that the guard on the train has that exact gun is extremely unlikely - he would most likely have had an officially issued sidearm.

