(as) sure as a gun

obsolete With utmost certainty; quite definitely. (That is, as sure as a gun is to fire when its trigger is pressed. This idiom has largely been replaced with the phrase "sure as shooting.") Lord Henry, sure as a gun it was those two miscreants who have made off with your prized heifer. Come into a great fortune and, as sure as a gun, the number of those who would be your friend increases tenfold. Edward has always been in love with Lady Mary, so, sure as a gun, he's the one who sabotaged your chances with her.