A 12 gauge shotgun with buckshot or a slug would be a (near 100%) successful way to go. Also make sure that you put the barrel in the mouth, angled as close to the back of the head for the highest chance of success. Make sure that you don't flinch or anticipate the recoil as that will result in unintentionally pulling the barrel away from your intended angle, thus increasing the likelihood of botching the attempt up. If you cannot obtain a shotgun or for some reason decide on a pistol, then a .357 magnum would be the way to go (if revolver) or 9mm hollowpoint bullets (if using a modern, semi-auto pistol), and aim from the side of the head, behind the ear to ensure that you hit the brainstem (personally, I'd still recommend the shotgun as that has a near 100% success rate, but very very messy). Whatever you do, do NOT put the gun under your chin, as that is the likelihood of failing a suicide via firearm and resulting in a vegetable and without a face (really bad condition to be in).
As far as jumping, I would make sure to get to a high enough place, the higher the better (and more likely you will succeed). Also consider the ground below (if it is concrete, grass, or water). If it is water, then you do need at least 250 feet or more to have a near 100% success rate. For solid ground, 150+ feet should be sufficient. Finally, do strive to have your head hit the ground first as that is the most fatal way to go.
