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I had a 30+ year old opened can of SR 4759...it was about 1/3 of a pound...so a fair bit of air space. This was the old metal can.

I also had a fresh 8 pounder and decided to do a test of the old vs new powder.

I loaded up identical loads in 45-70, only difference being one batch had new powder and one old.

The loads with the old powder were 131 fps faster (~7%!!).

That old powder was stored indoors and between A/C and heat, it evidently dried out a fair bit.

So, if you do load with really old powder that has been stored in conditioned air, you might want to make sure you start your load development a bit lower than you might otherwise.