Chrony Alpha

I've used one of these for about 15 years with no problems whatsoever, very reliable unit as long as you are careful not to shoot the sensors, LOL. I've never scientifically corroborated its accuracy, but I have tested it side-by-side with a friend's similar unit and they match each other well.

They have probably been vastly improved since then but it does require good daylight to work. The unit can remember a long string and show you the highest, lowest, average, and standard deviation for the group. I hand-write these for posterity, for a little more money you can get units that print this stuff out but I never particularly felt the need for that.

There are a few tricks to using it but you quickly get used to it. For example, you'd wanna keep a short-barrelled CO2 gun like your 150 back a few feet from the sensors to keep the gas cloud from distorting the readings, LOL.

I'm not a "speed freak" but I do find a chrono absolutely indispensable for airguns as a diagnostic tool. It will help you see if a gun is working right, and particularly by shooting longer strings (I use 15 shots typically), and calculating the SD, you will learn a lot of interesting stuff about how guns and pellets get along.