Yesterday I broke down a few factory 7mm Rem Mag rounds, some Win and Rem. The Win. was loaded with ball and the Rem with extruded. All the rounds were clean inside and out and nothing was contaminated. Not having enough for anything it is destined for fertilizer so I got to thinking (I know so no need to discuss that).
I weighed 100 grains of each and put in a small aluminum bowl and set out back under a shed. It didn't rain here but was cloudy and damp all day with humidity around 75%. I weighed both this evening after 24 hours outside, both weighed 100.4 grains. The ball powder was stuck to the pan but the extruded powder flowed freely.
Not very scientific but does show how quickly it draws moisture. I have never been one to leave powder in a measure but now I know it is a bad idea.