No suicide method can truly be called 100% certain. There are always unpredictable factors at play in life and in the body: someone might step in unexpectedly, a person's body could react in ways you didn't anticipate, or something in the environment might interfere. Still, in actual forensic reality, some methods are so close to being certain that survival is almost unheard of. When it does happen, it's usually a bizarre accident. For example, large-caliber firearms like a .357 Magnum, .44 Magnum, .500 Magnum or a 12-gauge shotgun aimed at the head have fatality rates that are practically total, with the very few survivors often linked to poorly executed attempts or split-second impulsive actions. The same goes for jumping from extreme heights of 200 to 250 metres or more, or being in direct range of a high-powered explosive. Statistically there is no realistic chance of walking away from those, though technically the odds are not a pure zero because of the extremely rare possibility of an unusual survival. So while saying "no method is 100% safe" is correct in a technical and scientific sense, in reality some methods are so lethal that the exceptions are almost meaningless.