This does bring up an interesting test that could be done.
Moisture level in the loaded cartridge vs velocity. You'd need a good way to measure humidity and the cartridges would need to be sealed once loaded.
Control of humidity could be done with something like a humidor.
Determine the humidity level for your 'control'. Weigh the powder at that level and load some up. Then take some of those same loads and put in high humidity. Let stabilize, weigh again (they should weigh a bit more now) and load. Another batch should be weighed after being in the high humidity and then loaded. To be a more precise test all of this should be done at a fairly constant temperature. About ten loads of each should be sufficient to see the effect, if any.
So you have three batches. One, control humidity level. Two, weighed at control level and 'soaked'. Three, weighed at 'soaked' level.
Could also do this with two different charges, one with a fairly low load density and one with a fairly high load density.
Test engineering is fun stuff!!!
[no, I won't be doing this
]
PS shooting should all be done at a constant temp, not one batch in the morning and another in the afternoon. Box of ammo should not be in direct sunlight, etc, etc. And, yes, the temp of the gun/barrel/chamber should be controlled and/or monitored.