I'm wondering if would be good to store ammo in a ammo can, with desiccant packs, inside a fire proof cabinet in the garage ? Will I have any issues?, ie rust, degradation?
I always store my stuff inside to avoid extreme temps. Bring all my stuff in for winter. If you shoot often and rotate stock should be good. Long term or serious use ammo (hunting/defense) I would bring it in.
With the anticipation of being flamed, my research and experience says that brass cased ammunition (short of being immersed) and powder is not affected by humidity levels you are likely to experience in a reasonable temperature controlled environment so I don't think desiccants are unnecessary but, certainly they can't hurt. Your worst enemy is high temperatures. I had many rounds of original factory loaded 45 ACP with a headstamp date of 1917 that were simply stored in a closet for 15 yrs after I acquired them (I have no idea how they were stored for the many years before). Just for jollies, I fired them in a Colt 1917 revolver back in the 80's with half moon clips and did not experience a single misfire. I have other stories about WWII surplus powder but, long story short, I wouldn't worry about it. (your mileage may vary)
I would think you would be ok as long as you don't have rapid temperature changes but the desiccant pack should help. I read the the desiccant pack is no longer good after it weighs 1.35 times it's original weight.
personally, i store all my loaded ammo, powder and primers in the basement. temps and humidity stay fairly consistent. my basement stays a few degrees warmer than the rest of the house in the winter, and a few degrees cooler in the summer.
i havent stored ammo in the garage since i left central florida. at the time, i didnt have much choice in where i stored & reloaded ammo.
I store mine in my barn inside a 60x60 Jobox type cabinet inside that they are all in Military ammo cans and have never had any issues at all even with temp changes.