You don't need to use it at it max range.
You can use this anti submarine missile at maximum range of your Ship borne sensors or air borne(both fixed wing and rotary wing) ASW aircraft sensors.
For example a frigate armed with SMART torpedoes and an ASW chopper, detects an enemy submarine 300 nautical miles from its position, using ASW chopper.
It can simply launch SMART torpedoes at the submarines co-ordinates, while staying well out of submarine's missile and torpedo range, as well its sensor range.