'Oh! Jesus Christ! I'm hit,' he said; and died.
Whether he vainly cursed or prayed indeed,
The Bullets chirped-In vain, vain, vain!
Machine-guns chuckled,-Tut-tut! Tut-tut!
And the Big Gun guffawed.
Another sighed,-'O Mother, -Mother, - Dad!'
Then smiled at nothing, childlike, being dead.
And the lofty Shrapnel-cloud
The gun is not only personified but presented as a proper noun. Its mocking ‘guffaw’ makes it seem more menacing. The gun is delighting in the men it mows down, without remorse. 
 
 It is interesting that Owen gives these weapons independent agency. Perhaps Owen was intending to exonerate the men from the responsibility of killing, given that they had to obey orders.