This is a simple spreadsheet that I use to calculate the muzzle velocity of my spud guns.  In order calculate this you must first construct a device called a pendulum.  Essentially what a pendulum is is a box suspended in mid-air by strings that swings side to side when hit by a projectile.  Using the weights and masses of the projectile, the pendulum and the gravitational pull on the earth.  If you are planning on building a pendulum, here are some instructions...

Begin by getting a small box, about 12" x 8" x 8".  Make sure that the rear is reinforced well.  Weight the box until it weighs exactly 64 oz.  Suspend the box from the four corners with strings that are __" long.  Suspend this box from some sort of ceiling.  I prefer to use a rig that involves the use of a ladder with 2 x 4's clamped on top.  the picture below should clarify.  Also notice the board set up behind the structure.  This board (I just use a white board) records how high the pendulum swings.  There is a marker strapped to the bottom of the box that draws a line as the box swings.

Before you go any farther, take a small scale and find the weight of the projectile that you plan on firing.

After you have this set up, get a few feet away from the box so you're sure that the cannon won't miss, and fire away. Watch how high the pendulum swings.  Measure the height or the rise of the line as shown below.

Use all of the information taken above, and fill in yellow blanks in the spreadsheet.  This will give you a very accurate muzzle velocity speed.  

*All the above taken from several sources.  Among others, Mr. William Gurstelle's work Backyard Ballistics, was a major contributor in this process.