12-gauge because it shoots with the biggest shells, slugs because the actual shot is just one big metal projectile (slugs are also known as the most powerful when it comes to shotguns and causes whatever it hits to explode on impact) but that's only a preferance because there's a very slim chance to survive with a shotgun eitherway, through the mouth so you don't miss, tilted upward to hit the brain stem aka the medulla oblongata (even if you actually miss the brain stem, it won't really matter because your head will be obliterated but it's just to increase mortality rate). The aspirin is to help bleed out faster if you do survive somehow, and so there's a smaller chance paramedics can save you.
I wasn't really sure what you meant by mechanics, but I hope my explanation sufficed. I had a lot of time to think about this procedure, so if there are any potential discrepancies in this plan, let me know.