We acquired a pair of 9.1-inch diameter, 90 pound, cored iron or steel balls to shoot from the mortar. We think the balls may have come from a ball mill such as is used to pulverize coal.
The mortar bore is about 9.5 in., so the difference between ball and bore diameters, 0.400 in., is the "windage."
Traditionally we'd like to have only 1/40 caliber, or 9.5/40=0.237 in. windage, so there will be about 0.163 in. more clearance than we'd like. Some folks in the North South Skirmish Ass'n. who we've been talking to, suggest just going ahead and shooting it with the 0.400 inch windage since all that will be sacrificed is a small amount of range.
We don't yet know how much balloting (ricocheting within the bore) there will be, so we're going to use either multiple large sheets of paper, such as newsprint, or some type of fabric, perhaps canvas, as a patch to get a little better fit, and protect the bronze bore from being damaged too easily. Our NSSA contacts concur with that, since it doesn't create any safety problems as long as the flammable patches don't set something on fire.
The photo shows the pair of 9.1-inch, 90 pound spheres we're planning to shoot.
For powder charge, we plan to begin with very light charges to see what happens. Then if everything looks good, we'll follow the range tables for the US 10-inch siege mortar M1861, since the bore diameter and projectile weight is very close to what we have. Since we only have two projectiles, we're only planning to shoot the balls as far as we can reliably recover them.
To Join the Company of Military Historians click here