Pape: Autocannons from IFVs and such can peel off (some) reactive armor, allowing tanks to get in a follow up shot. Also, most ERA is fairly difficult to trip off unless something big and nasty, like an RPG-7 round or above hits it. The high density shotgun round might not trip off every plate, reducing its reliability greatly. Something that can trip off ERA are AT cluster munitions fired by artillery or aircraft.