It would fall under a usbc rule about putting anything additional added to the ball.
From the General USBC manual for leagues
Plugs and Designs
1. Must be of material similar to, although not exactly the same as, the original material out of which
the ball was made and shall otherwise comply with all bowling ball specifications.
2. Plugs may be inserted for the purpose of redrilling a ball.
3. Plugs and grips may not exceed beyond the surface of the ball.
4. Designs may be embedded in a ball as guides, or observation or identification purposes, provided
the designs are flush with the outer surface of the ball.
In all cases there shall be no interior voids.
Altering a ball in any way, to increase its weight or cause it to be out of balance beyond the tolerances, is prohibited.
From the USBC manual for Equipment
4. No device shall be employed for the purpose of
adjusting the static balance of the ball.
5. No voids shall be permitted under the device.
6. Density not to exceed 1.5 grams per cubic centimeter.
No foreign material may be placed on the outer surface
of the ball. Nor may any foreign materials be
placed in a weight hole including but not limited to
tape, paint, or white out.
PAGE I-4 (April 09)
I hate trying to read through the rule book but it's in there somewhere.
Most cores aren't made of anything super crazy. It would be no different then using sand. The balls are drilled with masonry bits.
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