Green’s Dictionary of Slang

gun n.²

a flagon of ale.

1645		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.
1674		J. Ray Eng. Words Not Generally Used 23: A Gun, a great flagon of Ale sold for 3d. or 4d.
1729		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’ .
1785		Grose Classical Dict. of the Vulgar Tongue.

In phrases

in the gun (adj.)

drunk, tipsy.

c.1698		B.E. Dict. Canting Crew n.p. : In the Gun, Drunk.
1725		New Canting Dict. n.p.: He’s in the Gun; He’s in Liquor.
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. 1725].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.