[Ether]

Here's a fun little spreadsheet.

Put your launch speed, launch angle, and firing rate into cells L2, L3, and L4.

Column D will show the gap, in inches, between the balls.

If the gap is negative, the cell will be red.

This model just uses parabola equations for simplicity. Air drag is ignored.

You can easily select a relevant portion of columns B and C and graph it to see the balls appropriately spaced in mid-flight.