I like your theory more than anything I can come up with. It would require a head shot to take out Dimitri. If Brandon still has his finger on the trigger when someone else takes a shot, he might reflexively fire his shotgun. Let's hope not or that he misses Eric.
Also, there is an air force officer near the house. Most air force officers wouldn't normally carry a sidearm in the US, but he is an investigator, the military is on alert, and everyone else at the ranch is armed.