Hygroscopic qualities can be bad with some powders, less so with others. It can affect charge weight, but more importantly may lead to clumping, which I have experienced. If that happens, you may get a clump that partially to totally blocks the measure chamber so that you get erratic sub-charges. Besides poor accuracy it is a good way to get bullets stuck in your barrel then have to pull a bunch of loads. This is a "learn the hard way" example.
Also, I have noticed that leaving powder in my Hornady L-N-L hopper has caused it to swell. It is a hard poly type plastic rather than the type RCBS uses. Not a problem to fix, Hornady took care of it. But it is yet another reason to return powder to the can after you are finished.
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" I said, good DAY! "
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