Bright sunny day and cloudy day causes about a 100 FPS difference in my chronograph. Bright sunny day is consistent, where cloudy day is not. Sensor screens need a consistent lighting. I do ALL chronographing for load workups on a sunny day, and shield the sensor slots from any direct intrusion of sunlight, which also makes a difference.
A load worked up on a sunny day and then rechecked on a cloudy day will show a velocity change.
Low sun angles on bright sunny days are also to be avoided. The first couple of hours and last couple of hours are not for chronographing.
A precharched pneumatic air rifle that is pumped up to the same pressure every time is consistent in velocity with match grade pellets. Unlike firearms, as long as air pressure is the same it is the same, as measured by an accurate gauge. Air can be added or bled off if there is a big temp change. Air pressure is air pressure is air pressure. When pumping up a PCP, the air chamber gets hot, and pressure will reduce in a few minutes. I take a couple of extra minutes and add air as pressure drops as the air chamber settles to ambient pressure.
If you have a high quality PCP air rifle (Benjamin Marauder, BSA, Daystate, FX, Air Arms, etc.), then it is a very accurate checker of your chronograph.
A string of a half dozen shots will vary by about 3 FPS in my BSA Lonestar .25 cal.
Powder can vary significantly from lot to lot, but I have not found that to be the case with Hodgdon powders lately. Liability issues and modern manufacturing practices make canister powders quite consistent.
Primers can change from lot to lot . I bought a lifetime supply of WLR primers during a short time period some time ago. That's one problem I don't have to contend with. Winchester primers of today are not the same as years ago, just like their brass.
