Green’s Dictionary of Slang

squirter n.

a pistol.

1930		(con. 1910s) D. Mackenzie Hell’s Kitchen 117: A pistol is a ‘squirter’.
1935		G. Ingram Cockney Cavalcade 260: There’ve been reports round here of some of you fellows doing that film stuff with squirters and handkerchiefs over your faces.
1981		(con. 1900–30) A. Harding in Samuel East End Und. 284: Squirter – An ordinary pistol. ‘Be careful, he’s got a squirter.’.