The version most common in WW2, and similar to the version used in Korea which added a carry handle. This was the US army's attempt to make the BAR occupy the same role as the modern M249 or MG34 (no, really.)
colloquially known as the M1918A0 BAR, this was the first version adopted by the US military; it was the one fielded during WW1, the lightest and was most like a modern assault rifle.