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RG Bowling balls have a RG (Radius of Gyration) converted to a scale of 1-10. RG numbers range from 2.460 to 2.800, but some companies have converted them to a 1-10 scale to help give the consumer a better frame of reference. Mass distribution numbers describe the distribution of mass in the bowling ball. High RG numbers indicate that the balls mass is distributed more towards the cover (cover heavy) which promotes length through the heads of the bowling lane. Low RG numbers indicate that the balls mass is distributed more towards the center (center heavy) which promotes an earlier roll through the front part of the lane.
Please watch Talk Bowling Episode #0004 for a good explanation of RG.