The trivia item below may give away important plot points.
In the interview with Quentin Tarantino for an 'extra' in the 2008 re-release of the film on DVD Enzo G. Castellari points out that several scenes had to be re-scripted and/or shot differently as during filming the Italian Government, in response to increasing activity by the terrorist Red Brigade, drafted a law prohibiting the holding of weapons (even blank firing 'prop' weapons) in case they fell into Red Brigade hands. The castle rescue scenes originally featured a massive shoot out, but were rewritten into the covert attack and escape featured, using knives, catapults and crossbows instead of firearms. The scene where Bo Svenson's character shoots the guards in the train lab was shot differently with a single squib at the end of the (fake) MP40 in the long shot, followed by a zoom into Bo's face with the sound of gunfire dubbed. Enzo G. Castellari threw spent rounds into shot to maintain the illusion of the weapon firing with the set electrician waving his hand in front of a lamp to simulate muzzle flash.