Tracy,
Sorry for the long delay but with work, holidays & a new computer I have not been able to reply till now.
The attached photo should give u an idea how I aligned for each shot.
First, a line is struck to the target using two stakes and a string. Then the mortar bed is set next to and parallel to the line (string) then leveled. The mortar is placed on the bed using the reference lines in black. That way I can position it for each shot the same way.
If the first round falls left of target I move the rear stake slightly to the left say 1/2" then align the bed to again be parallel to the string.
Usually I can drop the second round within a few feet of the target.
Yes, I used "Mortar Mix" in all three projectiles. The tennis ball using the least amount but the heavier can fly truer. Less wind deflection.
Using the pin as u did is a good idea. I'm not a machinist so I did not think of that...
Santa brought me a digital camera so I hope to learn how to use it and post better pictures.
Happy New Year,
Wayne