Ok...first, everything except powder and primers are PROBABLY ok in the shed as long as they are dry.
That being said, if you are talking decades, you may want to think differently. At the very least, get the projectiles fairly warm in an oven to drive out moisture and then vacuum seal them with a desiccant thing, to prevent oxidation. I tend to go overboard and would probably seal them in motor oil...just use a gallon motor oil jug (CHEAP AS POSSIBLE) about 1/2 full and start filling it with bullets, making sure that the bullets on top are still covered in oil. Cases, same thing. It would not be difficult to remove the oil at some point in the future with any number of solvents, detergents, degreasers or good old elbow grease. I'd be sure to paint and mark and identify those jugs for posterity.
That oil doesn't have to be tossed, in fact, it would work just fine in the motor of your bugout/survival/getaway vehicle. Not everyone around the world has access to $50/ounce super dooper extra cool kid lube endorsed by every special operator in US SOCOM. Millions of AK-47's and other weapons are routinely and successfully cleaned with diesel fuel or gasoline siphoned from a trucks tank and lubricated with oil dripped off the dipstick and greased with stuff scavenged from the ball joints.
Powder and primers are slightly more difficult but before I tell you what I currently do, let me tell you about finding some primers in 1979 my dad had left in a box about 1969. They had not been stored in any sort of special way, just a box of 100 in with a bunch of other stuff. My dad was KIA Vietnam in 1969 so these may have been just tossed in that box by my mother. I was a senior in HS and didn't think much about any sort of degradation or oxidation or whatever and used them to make cartridges. No problems. A few years later, gaining some knowledge, I wondered how lucky I might have been but really, the primer by itself doesn't add greatly to the explosion.
I currently use the CHEAP white styrofoam coolers to store powder and primers in. I place the powder in the coolers while still in the unopened can. I fill the cooler as much as possible and place an oxygen absorber or 2. Then I use packing tape to seal the lid and tape it down, too. I use a Sharpie to label the cooler with what's inside. Then I place the cooler in a closet that does not have an exterior wall...the closet is 100% interior. The reasoning....temperature changes are the most detrimental to powder. especially rapid changes. Being in a closet, the HVAC system has little impact on the closet temperatures and with no exterior wall, that closet has a very uniform temperature though out the year.
Ammunition, treated much the same, temperatures held steady and humidity controlled, will last decades and decades.
