It is M109 an American-made self-propelled 155 mm howitzer, first introduced in the early 1960s. It was upgraded a number of times to today's M109A6 Paladin. The M109 family is the most common Western indirect-fire support weapon of maneuver brigades of armored and mechanized infantry divisions.
The M109 has a crew of six: the section chief, the driver, the gunner, the assistant gunner and two ammunition handlers.