Green’s Dictionary of Slang

(US) short, ‘sawn-off’.

1904		Wash. Times (DC) 25 May 6/4: For everywhere that ball club went along went Runty Joe.
1942		P. Wylie Generation of Vipers 108: I have seen runty Jewish Hollywood producers put on one of these hats.
1956		‘Ed Lacy’ Men from the Boys (1967) 126: One of these runty European automatics was in his right hand.
1968		R.D. Magoffin We Bushies 27: Now what the hell, ain’t it enough / To fight the runty Japs.