Thank you for the responses, it's much appreciated.
I'll be going through the firearms megathread soon.
I've read the entire thread.
12 gauge shotgun is the recommended weapon of choice. But in pistols, 9mm is the minimum recommended but many people suggest something like .357 magnum would be the better choice. The less power you have, the more you need to worry about accuracy. Shotgun will basically take your head off so long as you don't do anything foolish such as putting the gun under your chin. (please NEVER do that) And it should be instantaneous so long as you destroy the brainstem.
In the mouth is the top recommendation, aiming at an angle roughly in the direction of your ears (so if you picture an imaginary line drawn between both ears, you aim for the center point of that line) Behind the ear from outside the head is also effective, but not as easy to gauge accuracy. And not really a viable option with a shotgun/long gun. Only pistol. Heart shot is not really recommended by anyone.
Also, some other suggestions in that thread as a contingency against possible failure. Take some asprin, this will thin your blood and make you bleed out faster. Also possibly consider standing in waist deep water, so if the head shot doesn't instantly kill you (which it most likely will) then you should drown. You could also sit on a kayak and wearing something like a weight vest so your body will stay under. Also, practice your aim in a mirror without ammo. If it's a long gun, you might have to secure the butt of the gun against something or in a vice and pull the trigger with your toe... so you would be best to practice these things ahead of time so you're sure of what you're doing.
Also, if you're going to miss the brainstem, you're better to miss low than high apparently. The theory being that aiming too high might hit non critical parts of the brain, but missing a bit too low will likely still take out the spinal cord - which is almost the same thing as taking out the brainstem and will likely result in a quick death too. So favouring slightly lower is the smarter choice. Shotgun will obliterate everything even if your aim isn't perfect, so this is more of a consideration for pistols.