Practicing rock steady focus and cadence to your shooting helps remedy this. Attitude is probably more important than most realize. I enjoy races more if I'm not super competitive, but it also seems to help my shooting if I'm relaxed and feeling happy to be a part of the event. If I was super competitive during a race I bet I'd tense up, rush shots, and get distracted easily by other racers. We could all use a good attitude and encourage each other, for the overall success and continuation of Biathlon.
Heart rate is an interesting topic.
I remember as a kid seeing on TV a news piece about US Biathlon team members attached to heart rate monitors and using only mind control to lower their heart rate. This summer, I started practicing this during workouts, and within a couple months I could noticably change my heart rate. My ability is sporadic though. Sometimes it feels like I'm making my heart literally skip one or two beats. Other times I can lower the overall rate for a short period like 30 seconds.