Take a look at this post and a couple of those following to see how to determine what size balls to use.

You could have either 3 layers of 9 shot or 9 layers of 3 shot; 27 is 3 x 3 x 3 and no other possibility. 9 shot per layer would use .745" balls, 3 shot per layer would use 1.357" balls.

 

I think we are talking about two different things.
You are describing "canister shot" and calling it "grape shot".
Canister shot is multiple smaller diameter shot packed in a can in sawdust.

Grape shot is larger diameter balls packed around a center post and either tied up in a quilted wrap or held together with iron plates.

Note the difference in this picture.

Here is the grape shot I make.
For a 3" bore.

For a 12 pounder. Twenty five 1.5" diameter cast iron balls.
Quilted Grape.

3" bore and 4.62" bore.

These and more can be seen here on my website.
Zulu

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Michael's Gallery of Artillery Reproductions of Cannons and Cannon Carriages of the American Civil War and Revolutionary War items for sale. Replica black powder custom naval ammunition cannon ball artillery or cannonball projectiles, grape shot, cannister shot, cross bar shot, chain shot...

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