Long gun for sure, a firearm (unless you really really mess up, like hit the wrong area or miss the brain stem, etc.) is a surefire way to guarantee death and it's so fast that you will lose conscious and then die very shortly after (especially when you sever the brain stem/do enough damage to your brain that you die from blood loss, shock, etc.). A shotgun is really effective at doing so, but a rifle would work as well.