I tried concrete filled tennis balls in my 3"x3' mortar. I did not expect much as the 'windage' was very large. Well, it was very loud report and the ball disappeared into the blue sky, and I never saw where it came down. I was using an empty 22 shell box full of FF, with a fuse taped on. A soda can full of concrete with a chain link fence post finial bolted in would go about .5 mile with the same load. Both were at 45 degree elevation. The soda cans didn't make much noise at all, just a whoomph! I used masking tape to help hold the tennis ball in shape while it set. It was a waste of time.
Now I have a much better way of making concrete balls. I use mortar mix from a lumber yard. No pun intended. This method will make balls very close to the size of the tube you are using, so you will have to allow for your own windage, and the fact the balls are not perfect spheres.
Cut a piece of tubing 2/3 long as the inside diameter is wide. For example, if you wanted 3" balls, you would cut a piece of 3" tubing 2" long. . Cutting the tubing this long gives you the same volume as the sphere you want to make. (see formula at end if you are interested). Also cut a short ring of the tubing, say 1" long. This will be used to smooth up the ball. Clean the edges of the cuts with a half round file. You don't want any rough sharp edges. I used PVC pipe for mine but any round pipe will work. Spray some PAM cooking oil or some other release agent inside the tubes. Pack the long tube with a damp mud made from the bagged mortar mix. Don't mix more than you can use in a test run. Use just enough water to make it stick together. Don't make runny mud. Adding white glue or concrete adhesive will make the balls much stronger, which is important when they get up around 3". Now comes the trick part. Push the mortar out of the measured mold and roll it up into a ball, using your hands. Just like a snowball. Latex gloves are a good idea here, as lime will burn your skin real quick. If it is sloppy and drips, you added too much water or glue, so add more dry mortar mix and try again. One you have a nice ball put it back into the short 1" ring and it will fit, touching only at the edges. Spin the ball in the short ring and it will round itself out. Spin the ball in all directions so it is smooth on all sides. If the ball has low spots that are still rough, try adding a little more damp mix to the low spots. Any extra mortar will be extruded at the edges. This sounds harder than it is. Your problem now it to keep the ball from sagging and splitting as it hardens. I put them in the bottom of a 5 gallon bucket and put a plastic bag over them for a few days, keep them damp with a spray bottle. Checking them again in an hour or so with the short ring might be helpful. After a week I have dropped them on the floor and they don't break. The bigger balls are a challenge, but making a chicken wire cage skeleton would make it easier, but I haven't tried it. This isn't an exact 'how to', but it will give you the idea. Remember: they are not perfect spheres, so allow for it. You don't want one to wedge in halfway down the bore.
For the math majors: volume of a sphere
V=4/3*pi*r^3
1 1/3 times 3.14 times r times r times r where r is the radius or half the diameter of the tube (D/2=r)
volume of a cylinder
V=h*pi*r^2 where h is the height of the mold section
To get the height
set 4/3*pi*r^3=h*pi*r^2
divide both sides by pi you get
4/3*r^3=h*r^2
divide both sides by r^2 you get
4/3r=h
substituting D/2 for r you get
2/3D=h where h is the height of the fill up mold.
