I've had a bad experience with year-old triple 7. I'm using 95 grains with 295 grain bullet. I had a 50 yard shot at a deer the other day, and to make a long story short, it was hit in the front leg. (3 hours of tracking confirmed that) went to the target board and found that it was shooting 12 to 14 inches low. New powder put it back on zero. This powder was stored in my safe with no humidity problems and in the original container. from now on, I will not shoot year old 777.
So the ONLY difference was old vrs new 777? That is pretty amazing is was that much different after a year. I do try to use fresh powder for my M-L though, which is usually B209 now. I've also heard it stores better than most.
A number of years ago, the factory rep., told me that Pyrodex, will loose its power, if stored a year or more. I took a half can of powder, and a new unopened can to the range. The new powder went bang, with authority, the year old powder went off. Much less recoil, muzzle report. I would think that 777 would act similar.