Green’s Dictionary of Slang

gun n.3

[Partridge prefers to define this ‘a lie’]

a strange and unaccountable story.

[UK]Spiller in Anti-Theatre No. 13 38: Robinson Crusoe [...] has distinguished himself by many strange and unaccountable stories, which your smart fellows in conversation are pleased to call guns [OED].
[UK]New Canting Dict. n.p.: gun [...] a Lye.
[UK]Bailey Universal Etym. Eng. Dict. [as cit. 1725].