This variation of the FN Herstal (FNH) firearm brand is a modular rifle. The "SCAR" initial stands for "Speical Operations Forces Combat Assault Rifle." There are two main types of this class and it is known for it's large bursts of heavy fires. Both are available for Long Barrel and Close Quarters Combat.
Jack McClane is seen brandishing this weapon throughout the film as well as on various posters and DVD covers for the fifth film. John McClane also makes use of this weapon to take out some of Komarov and Irina's various henchmen in the closing scenes of the movie.
Jack McClane with a scoped version on the film's poster