Green’s Dictionary of Slang

gun n.2

[‘perhaps from an allusion to a vessel called a gun, used for ale in the universities’ (Grose, 1785)]

a flagon of ale.

[UK]J. Evelyn Diary (1850) I 213: We had a dinner of English powdered beef and other good meat, with store of wine and great guns, as the manner is.
[UK]J. Ray Eng. Words Not Generally Used 23: A Gun, a great flagon of Ale sold for 3d. or 4d.
[UK]Theobald in Nichols Illus. Literary Hist. 18th cent. (1817) II 246: I think there is a vehicle in the University, which they call a ‘Gun of Ale’ .
[UK]Grose Classical Dict. of the Vulgar Tongue.

In phrases