Benny, that 15 inch "grape" is actually canister I assume. It is interesting that the balls appear to be a bit smaller than those used in 12 pdr cansiter--27 balls 1.5" each.

As an interesting addition upon the 11 inch grape, from what I've found in two sources (the Confederate Field Manual and Coates "Cannons") the six pdr gun was supposed to use 3.58" shot--0.09" true windage on a 3.67" bore. On the other hand, Melton and Pawl's "Guide to Civil War Artillery Projectiles" shows a CSA solid shot at 3.57" diameter, a CSA shell at 3.57" another shell at 3.56" and one more shell at 3.55". Sounds like whatever the Federal standard was for 6 pdr shot, it should have been close enough to 3.55" that it would fit the 11 inch grape.