American Heritage Dictionary
(2)
Century Dictionary
GNU Webster's 1913
(1)
WordNet
(2)
Elsewhere on the web
Assemblyman Reed Gusciora's bill would exempt antique and muzzleloading firearms that use a traditional flintlock or cap lock and rely on black powder.— CourierPostOnline.com - News
Dave took the shotgun an' pap has grandpap's flintlock, but Billy didn't have a gun.— The Long Roll
He had an old flintlock, but in his dream he did not like it, and it changed to one of the beautiful modern rifles they were beginning to take from the Yankees.— The Long Roll

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You can expect to see this word about twice a year.