Factual error: Ronnie Cox is shot in the heart at point blank range by Brigitte Neilsen, and it looks to me like she's using a large caliber handgun. Keeping track of the action of the film tells us that Cox is sitting up in bed chatting to his friends just three days later. What nonsense. Even if he did survive (unlikely) he'd still be in Post-Op Intensive Care at that stage. He wouldn't be conscious, let alone allowed visitors.
Factual error: While Karla is shooting in the gun club she is using a revolver. Bullet shell casing are flying out from her gun and dropping to the ground. Revolvers do not eject the shell casings while firing.
Factual error: When breaking in at the gun club, Axel squeezes the gum foil in the door crack, and fastens it with the gum. But the gum is on the outside of the door. When he opens the door, the foil would flip a bit back, so it would not connect the magnet on the back of the door - where there is no gum.