I would never store shotgun shells or rifle cartridges in an attic. NO NO NO
I have shotgun shells and high caliber rifle cartridges boxes of them that I bought in the middle to late 1960's.
They are stored in my basement in a locked insulated wooden cabinet that I built for them.
There is a dehumidifier in my basement and I control the temperature and humidty of the basement.
All the ammo I have has gone boom with no miss fires, the new stuff and the old stuff.
On all the old ammo boxes 40 years and older that I have is says right on the box STORE IN A COOL DRY PLACE.
Get your self a ammo box and store them in a cool dry place.
All my hunting and shooting buddies do not store any ammo they buy it and shoot it all up at the range. If we are hunting and they have ammo left over they will keep it home for a week or two until they get back to the range next time and shoot it up.
STORE IN COOL DRY PLACE
Good Luck and keep shooting.