Ammonium nitrate by itself is not explosive. It requires some kind of hydrocarbon such as fuel oil, diesel or gasoline mixed with it to be an explosive. Tannerite could be the spark that sets that explosive off though because it comes in two parts and when mixed, Tannerite explodes when shot with a bullet.
So ammonium nitrate mixed with diesel fuel and put in a car with Tannerite could in theory be set off from a distance with a shot from a high powered rifle.