The rifle used by the sniper was a real working sniper rifle from WWII. The scope, however, was a modern replacement as the filmmakers couldn't afford a vintage one.
The M-16s used by the actors were plastic air-soft rifles. The shooting was so rigorous that by the end of the shoot, only one rifle remained in tact.
The director came down with a cold on the first day of shooting and was sick virtually the entire production. He spent one whole day directing from a chair, and another day directing while lying flat on his back on the ground.
The section of the fight scene with Lt. Floyd on the ground was originally choreographed with just one punch, but the scene was lengthened by cutting together hits from different takes, making the fight appear more brutal.
The film was shot entirely in continuity, which means that each scene appears in the movie in the same order it was filmed.