Complex sentence: The BAR was designed to be carried by advancing infantrymen, slung over the shoulder or fired from the hip, a concept called "walking fire"--thought to be necessary for the individual soldier during trench warfare.
Split into simpler sentences: The BAR was designed to be carried by infantrymen during an assault or advance while supported by the sling over the shoulder or fired from the hip. This is a concept called "walking fire"--thought to be necessary for the individual soldier during trench warfare.