First, you'll need 100 ammo for a shot. Beyond that, you need to hold down fire for about 4 seconds before the round discharges.
My advice (unless you're using the trainer or other means to replenish heavy weapon ammo) is to save it for bosses. Oh, and don't fire it too close to yourself or your squadmates. It's very much an area-of-effect weapon.
All times are (GMT -08:00) Pacific Time (US & Canada). Current time is 9:33:56 PM