Two points, if I may...

1. I've never been in a firefight where I had too much ammunition. You mention a semiauto with spare magazine. Why not speed loaders or speed strips with a revolver? I always carry extra ammunition in some form, either spare magazine or speed loader / speed strip. Are the chances slim to none that I'm actually going to need it? Yes. But I didn't survive this long playing the odds. I did it by being prepared, mentally, physically, and logistically, for virtually any realistic scenario.

2. A slow hit is always better than a fast miss. As others have mentioned, several different physiological changes occur during high stress situations. A firefight is a high stress situation. Part of being prepared for those changes is practice, practice, practice, and when you're tired of practicing, practice some more. If you can do so, the next time you shoot at the range, take a jog or brisk walk before shooting. Get your heart rate up and your breathing as well. It's amazing what just a little bit of stress will do to your ability to find and maintain a decent sight picture. Remember what that feels like, practice for it, and then practice for it some more. Yeah, it sucks, especially as we get older. Getting shot sucks worse.