This is the Holy Grail of combat shotgun issued by militaries for warfare, the M12 Trechgun was made in 1942. It is estimated that only about 1,400 were issued and far fewer are in collectors hands today. 

This rifle can be effective at ranges as far as 70 m (75 yards). The delivery of the large number of projectiles simultaneously makes the shotgun the most effective short range weapon commonly used, with a hit probability 45% greater than a sub-machine gun (5-round burst), and twice as great as an assault rifle (3-round burst).