That is true that there is no absolute, 100% failproof method (barring like getting nuked and/or meteor hitting Earth and dying instantaneously - which both are very, very improbable events at least in our current situation.), however, there are things that are almost guaranteed success if done correctly. Speaking of a 9mm or .45 ACP round, yes, those are more than sufficient for death, provided that you aim correctly. There is a guns/firearms megathread that speaks about using firearms as a method. It is one of the most reliable methods for suicide, provided you handle it correctly. NEVER aim underneath your chin, as that would result in a high chance of failure and also aim to hit the brainstem through the mouth (long gun) and that shotguns with 00 buckshot or slugs are very close to being failproof as long as you aim it through the back of your mouth close to the brainstem. I don't believe it would hurt unless you fail. Chances are when the weapon goes off, you will be unconscious and unable to hear/feel pain.