A really long shot? If you have a 1 moa rifle and your group is well centered, and you can judge the wind, and you are positive of the range, and the animal will hold still, you will hit within four inches of point of aim at 800 yards which most would consider a pretty long shot. Under field conditions, four inches is squat. Luck begins to play a significant part.
Hunting rifles of typical hunting configuration and in typical hunting chamberings, have limited accuracy potential; except on the internet, of course. So the primary reasons for owning the custom action become pride of ownership and /or a perception of exclusivity. Nothing wrong with that but long range performance is not the exclusive territory of the custom action.
As Butch mentioned, short range BR is the one venue where the use of a custom action verges on mandatory and it is rare that a serious competitor chooses to go any other way. "F" Class competition is beginning to get this way too. Hunting rifles? Not so much. GD