sure as shooting

(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.
See also: shoot, sure
Farlex Dictionary of Idioms.

sure as shooting

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 mid-1600s, a time when guns fired with far less certainty. [Second half of 1800s]
See also: shoot, sure
The American Heritage® Dictionary of Idioms by Christine Ammer.

sure as shooting

or

sure as shootin'

AMERICAN, INFORMAL
People say sure as shooting or sure as shootin' to emphasize that they are certain about something. Sure as shooting, someone will tell me what I've chosen is wrong. It's not the best computer, but it's sure as shootin' not the worst.
See also: shoot, sure
Collins COBUILD Idioms Dictionary, 3rd ed.

sure as shooting

Dead certain. This nineteenth-century Americanism may be a descendant of the older simile, sure as a gun, which dates from the mid-seventeenth century and appears in the works of Samuel Butler, John Dryden, William Congreve, and George Meredith, among others. According to S. A. Hammett (A Stray Yankee in Texas, 1853), American southerners drew the comparison from their much-loved rifles and rendered the alliterative cliché.
See also: shoot, sure
The Dictionary of Clichés by Christine Ammer
See also:
  • (as) sure as shooting
  • (as) sure as a gun
  • get (one's) teeth into (something)
  • get one’s teeth into something
  • get teeth into
  • get your teeth into
  • get your teeth into something
  • get/sink your teeth into something
  • get (one's) teeth in
  • sink (one's) teeth into (something)