12 guage shotgun with 00 buckshot, from all the research I've done, is essentially instant and painless. You pull that trigger and the lights are turned off. Even if you don't directly hit the brainstem, the shockwave and sheer energy of that blast alone will be enough to explode your head, rendering you totally unconscious.
With SN, you have to hope your dosage is correct, that you don't vomit, that your anxiety doesn't make you back out, and so on. Then there's the chance you could experience discomfort while waiting for it to work.
I prefer the good ole shotgun. It's tried, tested, and true for the last 450 years. Very, very, *VERY* few failures with this method, that often get overblown on sites like this.