I think I've got it. If someone forces entry into the barn, the RM may assume intent to cause great harm, and use deadly force, including firing a warning shot if the RM would rather not injure the burglar at that point. If the RM is driving around the ranch during the day, and spies someone trying to steal some equipment or livestock, the RM may point a firearm at the thief, and demand the thief leave. The RM may not fire a warning shot, but if the thief produces a weapon capable of inflict great harm, the RM will be justified in using the firearm to stop the threat.
What is the RM recourse if the thief ignores the demand, and simply continues loading up the equipment or livestock, without threatening the RM?