Guns are admittedly not my area of expertise because I'm from a country where it's very difficult to get one (but if it weren't, I probably would have been gone a long time ago), but as far as positioning is concerned, putting the gun in your mouth and positioning towards the brain stem should definitely get you an instant lights-out.
Also, I'm not sure if you're aware of this because you're a new member, but there's also an entire section on this forum dedicated to the gun method, under 'resources'. Maybe have a look at that, and I'm sure the people in there will be able to answer your questions FAR better than I can. Exit bags and hanging are more so my areas of "expertise", but I just wanted to chip in and let you know about the section on guns. which I think could be useful for you, and some of your questions may have already been answered there. In any case, regardless of what method you go with (if at all – it all boils down to personal autonomy, and I'm not encouraging that anyone do anything one way or another), it's something that needs to be well-planned and thought through, to ensure that this is really what you want and, if so, that you do it in a way that causes you the least amount of pain and suffering.
I'm just so sorry that you're in the position where you even need to ask these kinds of questions at all. I don't know what you're going through, but regardless, you have my full sympathy. Life can be terribly, unbearably cruel.