Take a sound recording (preferably digital) of you shooting at a target a few metres off. If you have the right programs you can measure the time between shot sound and impact sound.
This is a pretty good idea, and the only program that you would really need is any sound editor that lets you visualize the sound. But if you take a video, you can get both audio and video to measure the speed.
Video is a bit edgy - exposure times blur the projectile flight.
I was talking about increasing the frame rate of a camera so that you can see the projectile and get a more accurate reading. Many video cameras have different frame rates that you would use. In this case, you would want the frame rate to be as high as possible. I am sure that anyone can get a camera into focus, and since you would position the camera parallel to the path of the projectile, its depth would not in respect to the camera. If you know what you are doing, you can easily make the video work.
Speed = Distance/time
v=dx/dt (velocity equals the change in distance over change in time)
If it is that simple, why do you need a computer to do it for you? Honestly, If you give the projectile a long enough distance to travel, you could do this with a stopwatch and tape measure (though it won't be as close to "muzzle velocity" as the other tests). Measure the distance and the time that it took the projectile to go that distance, and then divide the distance by time. It really is that simple.
The Drenchenator, also known as Lt. Col. Drench.