No, you took the wrong message from the thread.
A 12ga shotgun is indeed a reliable method, using no less than #4 birdshot (larger, such as buckshot, is generally considered to offer better margin for error), but so is a handgun of adequately large caliber --no smaller than .38 Special in revolvers or 9mm in semiautomatics, and always use hollowpoints. If you do choose a shotgun, I advise against a double-barrel, as it is more difficult to fit it in your mouth, and you'll only be using one barrel anyway. (Double-barrel shotguns fire one barrel at a time; to that end, they often only have a single trigger.)
Aim is the most important factor: you must hit the brainstem to die. To do that, do not aim the gun at your temple or up from underneath your chin. Aim into your mouth, pointed straight back and angled slightly up, or behind your ear, aimed level and straight across side-to-side. There are diagrams in that megathread. The larger the caliber you choose, the better margin for error you have in case you miss the brainstem.