I have one of those £20 ones, it's OK if you want a quick reading. Shot strings are another matter as I can only speak for mine, after about 15 shots it freezes and you have to unplug it and wait for the memory to die, which is about 3 mins. Very annoying, but got me out the poop when setting the power a few times.
Again in meters per second which is a 3ft per second variable either side, but tested against a combro and accurate enough.
Would I recommend, yes if for a test, no for serious work
Super soaker 3000 (water), nerf fang (foam), noisy cricket (energy), m41a pulse rifle (10x24), Gat gun (.177)