One Piece Marineford War
Quoting: According to Oda, Beckman has the highest IQ of all the characters introduced in the East Blue saga. Since he is a first mate to a Yonko, he presumably possesses a great deal of strength. During the Battle of Marineford, Admiral Kizaru chose not to fight him, even though he had just fought the First Commander of Whitebeard's crew and Whitebeard himself. We see Marguerite attacking Luffy with an Haki infused arrow here: Benn Beckman being the first mate of the Red Hair Pirates obviously has incredible power and Haki. Since Kizaru as well backed off, it makes it evident that Beckmann can imbue his bullets in Haki as well, although that hasn't been directly shown yet.
The Battle of Marineford (マリンフォード頂上戦争, Marinfōdo Chōjō Sensō?, literally meaning 'Summit War of Marineford') took place on the island of Marineford and the end results were that the Whitebeard Pirates failed to save Ace from death and lost their captain Edward 'Whitebeard' Newgate, along with many other comrades. 
Why was Haki not used during the Marineford War, as every vice-admiral. In One Piece, if someone used Armament Haki to cover an entire gun, would that.
One Piece Marineford War Full English Sub
One Piece Marineford War English Dubbed
Another reason would obviously be to avoid engaging in a war with the Red Hair Pirates/one of the Yonko. The revenant movie trailer. We already saw the kind of power the Yonko have when it took the whole of Marine Power to subdue Whitebeard.
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