A 10 ga. technically is measured by casting lead balls in the exact diameter of the bore and counting how many balls it takes to add up to a pound. In the case of a ten guage, that would be...lemme see, gotta do the math...sixteen ounces to the pound, one tench of a pound, carry the 7, subtract the exponent, square the compound interest, the train leaving Philadelphia was travelling south at 44 miles per hour...aw heck, you figger it out.
Something like 1.6 oz, or 700 grains.
Typically, in these bigger bores, folks like to choose a roundball about 15-20/1,000ths smaller than the actual bore.