Relating to older balls.. (pre mid 1990's).. I find today's covers with 4000 abralon are still atleast 2x as strong as the older balls that are at 400-800 grit US (700-1500P Abralon Standard).. If you take an older ball to 4000 abralon it seems to turn it into a marble..
That is what I have seen.
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16-17 mph,350 rpm,PAP 5 1/2 x 3/8up, HighGame 300 x 4, High Series 808
Book Average 205,PBA Xperience ave180