I found this reference to "bust a cap" in a 1951 Gene Autry novel. Gene Autry and the Golden Ladder Gang by W. H. Hutchinson. Whitman 2349
"I guess it is," said Pres as he regretfully fondled the stock of his rifle, "but I surelee hope that somebody gives me a little excuse to bust a cap in their direction."
-- Gene Autry and the Golden Ladder Gang 1951
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