[Scott in Texas]

If you are storing ammo in a hot, humid environment then yes, you are ruining it.
Lead will frost, powder will degrade and brass will tarnish.
I live in Houston, similar climate, and all ammo, powder and primers are inside. I would never store any of it in my garage. I've seen what happens to tools, plastics, chemicals etc after 5-10 years.

 

[Scott in Texas]

When I am reloading I make sure no primers are left in the Dillon 750 and I empty the powder measure back into the container. I won't leave either in the garage overnight.

A friend who lives near me left primers in his Dillon for a couple of weeks and he had multiple primer misfeeds until he started using new ones directly from his house. The humidity is just a major issue.

 

[Scott in Texas]

Pretty sure most milsurp has lacquered primers...and it becomes 'surplus' when it gets sketchy due to poor storage.Why else would they offload it?

I had some 1980's Malaysian surplus 5.56 that i bought mid 1990's. Looked terrible. It fired but cases were seriously tarnished and I unloaded it during the Obama Panic.

 

[Scott in Texas]

Wood is a great insulator . Your garage floor probably stays in the 70s . You can sketch up a box with a hinged top out of 1” x 8” boards. lowes will cut pieces for you. Use a thinner piece like 1/4” on bottom. Assemble. Store ammo on floor in box. The contents will stabilize some where btwn air temp and floor temp. Even easier might be to use one of those wood cases commie ammo comes in although it might not work as well as a thin bottom box for contact coolness with floor. You can even use some of that thin packing material from staples for added effect over the ammo inside of box though not on bottom.

How does this mitigate the effects of 90% humidity? Even my FMJ projectiles tarnish after a few months.

 

[Scott in Texas]

For those who keep a truck/car gun. How often do yall cycle out the munition?

I recently fired 3 mags of Remington Golden Sabre that had been in my car 'lock box' for about 24 months. All 30 rds functioned perfectly. I was hoping for that but was concerned enough to test and replace.

Projectiles were tarnished but cases were nickle-plated and pristine. Not sure brass cases would have been and that's a potential chamber / extraction issue. But that's why I use RGS in my car gun.