Simple way to measure fps using a camera and cardboard. your video camera takes a picture several times per second, if you know how fast id dose this then you can calculate speed my camera takes a picture 30 times per second (30 frames per second) if your camera is slower it will be less accurate and might not see projectile.
I shot my 180 fps airsoft gun and my camera got only 1frame of it. My 270 fps gun the camera did not see. this is not to be judged in the launch it competition, its for the many people that could use this to improve their instructablesvideo on step 5
using some gestimation (fired sling shot and half way pulled back) i estimated the speed of my sling shot
at 72 miles per hour (at full speed the camera didnt see any thing so i just multiplied by 2 i know its not accurate but it dosent have to be.)
the pics below:
it spent 4 frames on the lined part of board. after crunching the numbers the 14 gram projectile averaged 7.5fps (this is one of 5 tests) it was fired from my 7 gram torsion spring catapult