Squid, I've had a shotgun pointed at me from 25' away, and let me tell you, it really starts the adrenalin going!!! At that point, I pulled my duty weapon (I was a security officer at the time) and it ended up being a standoff until he dropped the weapon. I've also been in one situation when I had to pull my CCW piece, but didn't have to use it as the potential assailant stopped in his tracks and went away. That got the heart rate up, too. I had all the slack on the triggers taken up in both cases. We've all seen video of trained cops having to shoot, firing 5 rounds, and missing with all 5 due to adrenalin and the perp moving. I think my first shot would come slowly (assessing the threat), but the next would be in rapid succession if the threat was still there. The first slow shot would allow me to assess, get a good shooting grip/stance, and aim carefully unless the situation developed too rapidly for that. Our CWP instructor told us we might end up just pulling our piece and shooting like Reed and McCoy on Adam-12, from the low-grip stance. One-handed, even, or cross body; and he suggested we also practice these methods until we were comfortable with them.
All any of us can hope for is we calm down and stop shaking long enough to negate the threat effectively.