1
general::
Most certainly, as in It's going to snow tonight, sure as shooting, or That grizzly is sure as shooting going to make dinner out of us. This idiom has replaced the older sure as a gun, dating from the mid1600s, a time when guns fired with far less certainty. [Second half of 1800s]
American Heritage Idioms
2
general::
Phrase(s): 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!
McGrawhill's American Idioms And Phrasal Verbs