I would say if it's a shot to the heart there's a good chance you're only going to get one in. And if you don't have a good sentimental reason to do a shot to the heart. I advise shoving the gun into your mouth and point it upwards. However, one shot to the heart should kill pretty fast if your aim is right. Even if it isn't, if nobody comes to your rescue you will most likely still bleed out, but that would mean a painful death. There's also the chance that you miss the heart, and have chronic pain afterwards because someone rescued you.