Whichever chronograph you get (I have an Alpha Master Chrony myself,) you will need to make sure you use it properly in order to get good, consistent results. The principle of operation is very simple, the chrono measures the elapsed time from when it detects the bullet at the first sensor to when it detects it at the second sensor. The sensitivity of the sensor is where people have problems with chronos. I find that my Chrony works best on cloudy days without the diffusers. I use the diffusers when it's a bright sunny day. But that's a small issue.
The elapsed time between first and second sensor is VERY short. On a Chrony, the sensors are 2 feet apart, IIRC. So for a bullet going 3000FPS, the elapsed time over the sensors is in the order of 1/1,500 of a second .0015 seconds. The Chrony assumes that the distance covered by the bullet is the shortest distance possible between the sensors, which is 24 inches. If you do not open the Chrony completely flat, the distance between the sensors will be shorter so your velocities will be higher. If you set your Chrony at an angle, in other words not perfect level, the distance traveled over the sensors will be longer and your velocity will be slower. If you do not shoot your bullet perfectly level through the sensors and at the same distance from the middle line your velocity will be slower. Also, you need to set the front sensors at exactly the same distance from the muzzle every time, if you want to have repeatable measurements. On my Alpha chrony, I used a marker to indicate 12 feet from the device. I use a camera tripod with a bubble level to make sure it's set properly and I shoot level to the ground through the middle, about 4 inches above the sensors. Every time. Whatever you do, you need to repeat every time to get consisten readings. Have fun.