Re: High extreme spread
If you chronograph on a day with widely varying light conditions, it will affect the accuracy of chrono results. I learned to chrono ammo of a sunny day with no clouds between chrono and sun. Also, a high summer sun shining on the chrono sensor will affect the reading as well. A cardboard shield taped up high enough to just keep out direct sunlight from entering the sensor cavity will solve this problem.
I have seen chrono readings jump about 100 FPS between direct sunlight on the chrono and then a dark cloud passing over. I use the Shooting Chrony F1 Master, and it will apply to all similar Shooting Chrony chronographs.
Amount of cloud cover is not consistent, but a bright sunny day is consistent. Also, the sun should be 30 degres or more above the horizon to have a consistent light source for the sensors. No chrono readings taken early or late in the day for me.
To test a chrono for consistent readings, use a high quality precharged pneumatic air rifle and shoot weight sorted pellets through it. The air rifle should be fully regulated or self regulated with fill pressure set at best accuracy range. With my BSA Lonestar .25 cal air rifle using JSB pellets, a string of shots will be within about 3 FPS range. High quality PCP air rifles are amazingly consistent in velocity if the pellets are match grade. JSB pellets are usually within about a .1 grain spread. So good that I quit weight sorting them. So buy a can of JSB pellets and have somebody with a fully regulated or self regulated PCP air rifle shoot over your chrono.
Self regulated means the velocity is consistent over a range of shots, maybe as many as 10 in my air rifle, if you keep fill pressure in a narrow range. Hunting rifles tend to be self regulated while target rifles tend to be fully regulated. The fully regulated rifle suffers power loss as the regulation is set at a much lower pressure than fill pressure, but the pressure to the barrel is constant shot to shot until fill pressure drops to regulation pressure. A regulator does not increase pressure, just drops a higher pressure to a set lower pressure. Hunters need full power, so something like a Benjamin Marauder is self regulated. An exception is the Benjamin .357 Rogue. It has an electronically controlled air valve that lets you set energy anywhere you want it, provided you have enough fill pressure, all the way up to 250 FPE, the same as a .38 Special revolver. Definitely a hunting air rifle, designed for animals up to deer size. Find a guy with one of those, and it will be a very consistent shooting rifle that is useful for chrono testing.
Now, after confirming you have a consistent rifle and round, shoot over your chrono on a day of sun and clouds and watch the readings change quite a bit. In this case, it is your chrono and not your load that is inconsistent.