Roto Grip Bowling Balls. The Mercurytm is an entry level ball energized by the newly designed Orbitertm core. This high RG low differential weight block delivers increased rev rate in the mid lane along with smooth continuation down the lane.Encapsulating this new power plant is the always successful Sure Grip IItm pearlized reactive coverstock. This mild reactive blend gives you fluid acceleration through the heads and dependable backend motion through the pins.