Three days into a Gulf Coast outage the house is hot, the air conditioner is dead, and the ammunition you bought two summers ago is in a plastic tote in the garage. Nothing dramatic happens. That is the point. Ammunition does not fail loudly. It fails quietly, as a misfire when you need it or a group that opens up because the powder has been cooked.
The Sporting Arms and Ammunition Manufacturers Institute is direct about the mechanism: factory ammunition functions from arctic cold to tropical rainforest, but extended exposure to high heat or high humidity may damage it, and that damage can show up as increased or decreased chamber pressure, incomplete burning of the propellant, or a failure to fire. The SAAMI storage and handling recommendations are worth keeping offline.
Two things, both more important than the outdoor temperature. Your air conditioner is also your dehumidifier; when it stops, indoor relative humidity climbs toward whatever it is outside, and a Gulf Coast summer supplies plenty. Heat also stratifies badly, so an attic, a garage under a west-facing roof, or a metal shed runs far hotter than the room you stand in. A multi-day outage turns those spaces into ovens.
A third problem, less obvious: many safes use a low-wattage heating rod to hold the interior above the dew point. Unplug it and the safe becomes the most humid enclosed space in the house.
The week before a named storm is the time to relocate. Interior closet, interior wall, ground floor, off bare concrete, away from the water heater, gasoline, and solvents. SAAMI recommends a cool, dry location away from heat sources, with ammunition stored separately from firearms and inaccessible to anyone who should not have it. Staging a firearm you can actually reach after dark is a separate problem, covered in the home-defense long gun piece. If the power is already out, do not haul a heavy crate up a dark staircase; move what has to stay dry and finish when the grid is back.
A hygrometer in the container costs less than a box of ammunition and tells you whether the system works. Rechargeable silica changes color when saturated and resets in an oven. The common mistake is sealing a can mid-outage, when indoor air is wettest, then trusting it for five years. Seal when the air is dry and cool, and write the date on the lid.
Mark month and year on every container, new purchases behind old ones. SAAMI also recommends emptying a magazine rather than topping it off so older cartridges fire first; repeated recoil and rechambering damage cases and can push bullets deeper into the case, raising pressure. If a stored batch stops grouping, check the bore and the load before condemning the ammunition, since a bullet weight and twist mismatch looks identical, which is what twist rate actually changes.
Green or white powdery corrosion on brass, pitting on case heads, split necks, deep dents. If you reload, powder that smells sour or clumps in the measure is finished. At the range a weak report, a misfire, or a bullet that never leaves the barrel means stop and clear the bore before firing again. SAAMI is blunt: inspect before use, do not fire ammunition that is damaged or suspected of being damaged, and dispose of it under local regulations.
SAAMI notes that one primer detonating in a pile of loose primers will normally set off the rest in a single violent blast. They stay in factory packaging, never in a jar or coffee tin, and stay dry below 150 F. Do not store powder and primers together, and check local fire code limits before building a stockpile. The SAAMI primer guidance spells out the handling rules.
If a wet batch has to be replaced fast, shipping takes days and shelves empty before a storm, so find an FFL dealer in Texas that stocks ammunition now rather than during the outage.
The logic from the blackout lighting piece applies here: the gear you can find and trust at 2 a.m. is gear that was organized beforehand. None of this is exotic. A can, a pack of desiccant, a cheap gauge, and the habit of writing a date on a lid. When the air conditioning stops for three days, that is the difference between ammunition you trust and ammunition you are guessing about.