This subject has come up a few times and to those of you who have heard it from me before I apologize. I am not an engineer, an active reloader, or a rocket scientist. With that disclaimer let me add a few things. Changing temperature changes humidity. So avoid that. It's better to maintain constant temp than to control any other variable. A little more heat is better than varying temp--within reason, if there is any such thing left in the world.
I have read that shells with a hard crimp on them are more resistant to the vagaries of atmosphere. I have no data to support that assertion. If it is true, then .22 rimfire and many pistol cartridges are more vulnerable to deterioration. High-powered rifle ammo would be the most "bullet-proof."
I purchased one of those vacuum seal plastic bag units at Costco for my Y2K prep, and decided that it would be great for my ammo, controlling the humidity problem that I have in my basement storage room.
Well, the food I bagged is still good. The jury is still out on the ammo. I can say that the Rem. .22 ammo bricks that I stored in Zip-Locs with dessicant have had a few duds, which I've never had before with this ammo. Bad quality control? The target ammo, RWS and Federal, stored in the shrink-wrap, have been flawless. But then, when one brand costs six times as much as the other, one might expect a higher standard of quality control.
The CF stuff is hard to gauge. I put dessicant bags in three .50 ammo cans of .223 from Georgia Arms ( A great company by the way) and stored them in the same room, with variable temp and high humidity. The cans sealed well (they make that sucking sound when opened, like a good vacuum should). All ammo shot well, no malfs, selected at random, for 500 rounds. At least that's what my godson told me when he got back from the range with my Mini-14. How can anyone shoot that much, anyway?
By the way, if you try to store pointed bullets in the shrink-wrap style, line them up carefuly. Or put them in another container. A few attempts with my .223 stuff had the bullets piercing the plastic and breaking the seal--creating no seal whatsoever.
It seems logical that the home-style shrink wrap stuff would keep your ammo good for a long period, even buried in the ground if you were so inclined. But that's just my opinion. America; tell me what you think. By the way, Dennis Miller's rant on gun control really stank. Maybe now that we have him doing broadcast TV, we should tell him a thing or two...
Good luck to you.