Grape shot was the larger of the two types with the shot running an inch or more in diameter. It's also made of iron. Canister was smaller, with the rounds being about the same size of a musket ball (a bit over half an inch in diameter).

As usual, these pictures take me on other journeys in education and I found this site: http://www.bigcountry.de/index.php?Seite=/artillerie_2.htm
which has an illustration of how these things are put together. The site is in German I think but you can see from the pictures what they are talking about. Very interesting. I had no idea a cannon ball was so deadly. But then, I'm just an old girl learning new tricks.

Also

http://www.civilwarartillery.com/default.htm

I am from Arkansas. The particular conflict was referred to by my grandparents as "The War of Northern Aggression." Since their great uncles were veterans, and they knew them well, it was still a relevant event for them. This was passed down to me, and geez, I was born in the early 60s. The South has longer memories.

To the left of center, in front of the man standing on the piled roundshot, there's a whole mess of canister shot or grape shot (I'm not sure which).
In either case, when fired, they acted like giant shotgun shells and were gruesomely effective against massed troops.

From my contact with friends and family in the Deep South I'd guess those men in the arsenal were much more likely saying, "Save these bricks and your Confederate money, boys, the South's goin' to rise again!"

On a recent trip to Charleston, SC our tour guide whose family had fought for the Confederacy made a point of telling about "what the Yankees did to us." It was still personal after 150 years.

State of the art in weaponry. Cast-iron balls thrown a great ways with a kick of gunpowder. I like how the chaps in the background have been piling up the bricks from the fallen buildings - bricks not only stand the test of time and fire, but are reusable. "Well fellers, might as well start pilin up them bricks..."