Actually, if the container has been opened, you were given containers of "X" where content inside is unknown.
Reloaders are human and humans make mistakes. Some reloaders have been known to keep empty 1 lb containers to transfer powders from 4 and 8 lb containers. A few reloaders have also been known to pour unused powder from powder measure into wrong 1/4/8 lb containers.
Bottom line, you cannot be sure of contents of opened powder containers. When in doubt, do not use the powder and scatter on lawn or garden as fertilizer.
Faster powder burn (Higher chamber pressures) than when powder was originally manufactured. And if different powders were unintentionally mixed and they started to deteriorate, even worse.
And just like uncertainty of contents inside opened powder containers, you can't be sure of anything even though you were told powder containers were stored at such and such temperature for such and such number of years. Even though you were told the powder containers were stored there for 5-7 years, possibility exists some of them could have been stored longer (perhaps at higher temperatures inside a garage/storage shed) then given to the person who placed them on the window sill.
Bottom line, you just can't be sure of used powder as to how they were stored, particularly to temperature. When in doubt, do not use the powder and scatter on lawn or garden as fertilizer.
This is from Hodgdon on "Gunpowder Stability" -
http://www.hodgdonreloading.com/reloading-education/safety/gunpowder-stability
"Smokeless Propellant
The main ingredient of smokeless propellant, comprising from about 55% to 90% of the composition, is nitrocellulose. The process of creating nitrocellulose leaves remnant acid in the material. This acid immediately starts decomposing the finished product. Left alone the decomposition will reach the stage where the propellant becomes unstable and self-ignites. This process resulted in massive explosions at U.S. Government arsenals after World War I.
To increase the life of the smokeless propellant, a stabilizing chemical is used. This 'stabilizer' reacts with the acid to slow down the decomposition process. However, as the stabilizer reacts with the acid it is consumed. After the stabilizer is totally consumed, the propellant is no longer protected from the internal acid.
The entire stabilizer / decomposition process is a time and temperature function – the higher the temperature, the shorter the safe life of the powder. Even moderate temperature, over extended time, leads to propellant decomposition. As a rule of thumb, any temperature over that which is comfortable to a person is accelerating the decomposition of smokeless propellants.
Under proper storage, modern smokeless powder can last for decades. However, this does not mean the reloader can ignore how the powder is stored, particularly if in an uncontrolled environment such as a garage or storage building."
Keep in mind, higher storage temperature significantly shortens powder life. Below is a table from CMP forum and addresses dangers of using deteriorated powder (Note, significant degradation starts above 86F) -
http://forums.thecmp.org/showthread.php?t=157820
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