In the US they can shoot any caliber. Canada with it's archaic ideas that we need for follow the "Bisley Bible" or follow ICFRA Rules have been loosing TR shooters for decades and gaining very few. Time to wake up. Not everyone cares about making a Bisley Team or a Palma Team so let them shoot want they want (within the range template restrictions). If you are intent to shoot on an international team then make .308Win/155gr bullets the requirement to qualify. Also note the teams to the West Indies require shooters to shoot .308.
Who knows, maybe in another 25 years they may allow optical sights in TR.