Well, in my opinion, no shot is "100%". However, a head shot under 50yds with either my 30-30 or my AR-15 is going to be 98% for me.
Most deer that are injured or lost were shots that never should have been taken. And yes , in my younger days, I made a few of these mistakes myself. Been a long time since it's happened to me, but nothing more frustrating than tracking for hours and not finding the animal, and then 2 days later the buzzards show you where it is.