I think SI is going to be inherent in ANY method you choose. Just as you may be unable to "pull the trigger" with a firearm, you may, also, be unable to jump from a tall building, stick your neck down on the railroad track, drink the glass of Nembutal or SN, kick the chair out from under yourself while the noose is around your neck, enter into your tent that is filled with CO gas, etc, etc, etc. It's not the method causing the SI; it's about the result that the method may bring about. I really don't believe in "souls" per say, but I do separate the body from the "consciousness", which is you. The body doesn't want to die and has evolved it's own mechanisms of survival over thousands of years. You have to overcome the body's innate ability to protect itself. It's hard.
Just curious....what caliber gun did you decide on? Handgun?