(as) sure as shooting
With utmost certainty; quite definitely. (That is, as sure as a gun is to fire when its trigger is pressed.) It had to have been Mike who leaked our plans to the press, sure as shooting! Come into a great fortune and, as sure as shooting, the number of those who would be your friend increases tenfold.
Sure as shooting!
Inf. Absolutely yes! (An elaboration of Sure.) Bill: Are you going to be there Monday night? Bob: Sure as shooting! Bob: Will you take this over to the main office? Bill: Sure as shooting!