Green’s Dictionary of Slang

(US) a .22 calibre pistol.

1924		D. Hammett ‘Night Shots’ in Nightmare Town (2001) 81: A twenty-two and my forty-four — which I keep in the car — are the only pistols in the place.
1934		R. Chandler ‘Finger Man’ in Pearls Are a Nuisance (1964) 108: I got my twenty-two, which is all the gun I have, and hid in the bushes.
1942		N. Algren Never Come Morning (1988) 149: Which of your guests filed the numbers off that twenty-two?
1969		H. Rap Brown Die Nigger Die! 17: Even a lot of these so-called ‘militants’ go around pulling their 22’s on Black people and ‘tomming’ when the white man comes around.
1979		S. King Dead Zone (1980) 283: I gave him the .22.
1986		C. Stroud Close Pursuit (1988) 244: We’re trying to lift a latent off a twenty-two.
1993		E. Richards Cocaine True 78: The guy pulled a .22 from the side of the car.
2000		C. Cook Robbers (2001) 224: Only he carried a thirtytwo, not a twentytwo.
2005		L. Theroux Call of the Weird (2006) 192: They would send a goon squad to [...] finish him off ‘execution-style with a dirty twenty-two.’.