The humidity level of propellant/powder has a major influence on its burning characteristics, which ultimately controls bullet velocity, and therefore affects accuracy and trajectory. By placing powder in a controlled humidity environment, there is a real gain to bring more stability to your loaded rounds by decreasing powder fluctuations.
We believe this is one of the best bang for buck steps a reloader can add to his process to obtain more consistent data and results.
Powder manufacturers recommend that it is always beneficial to control the humidity of powder during storage and use. Consensus are that the powder humidity should be kept between 30% and 60%;
The bullet velocity variance of 2.8 ft/sec could be even much higher when outside of the 30% to 60% humidity zone;
This Potassium Carbonate (K2CO3) maintains the humidity at a level of about 43% when placed inside a sealed container.