While avoiding humidity is pretty much a universal theme on the topic of long-term ammunition storage, there is a lot of directly contradictory information when it comes to heat. I’d like your input if you have any experience or observations in this area.
The generally very informative Ammo Oracle at AR15.com has this to say:
The three primary killers of ammo are heat, moisture, and chemicals. Excessive heat will break down both the powder and the primer compound over time, causing erratic ignition and velocities.
Military guidance on how much heat is too much heat doesn’t offer much. Department of the Army Pamphlet 700–16, THE ARMY AMMUNITION MANAGEMENT SYSTEM for Logistics, does not list any temperatures, noting only that:
In general, ammunition must be protected from moisture and should be protected from temperature extremes when feasible.
Department of the Army Pamphlet 385-65, Ammunition and Explosives Safety Standards, says in Chapter 2, Field Storage of Ammunition:
When standard magazines are not available, buildings used for AE [ammunition and explosives] storage must afford protection against moisture and excessive changes in temperature and have means for adequate ventilation. . . The AE containing explosives or combustibles (such as, black powder, tracer composition, or pyrotechnic mixtures) that deteriorate rapidly in damp or high temperature environments should be stored under the best cover available.
And MARINE CORPS ORDER P8020.10A, MARINE CORPS AMMUNITION MANAGEMENT AND EXPLOSIVES SAFETY POLICY MANUAL, does not refer to heat or temperature at all, in regard to ammo storage.
Anecdotally, some say heat is bad, others note they’ve used ammo stored in very hot bunkers with no issues. Various old and/or surplus ammo from the 1950s or earlier not stored in cool conditions that still functions, and hand-loaded .223 ammo in a VA attic for ~25 years, working fine.
You can find dozens of forums or blogs that say one other, but I haven’t seen any actual studies. From a common sense perspective, seems like heat might affect ammo since it affects food and many other chemical compounds. But with a little more information, a lot of real-world examples seem to point to heat not mattering much, which might alter what passes for common sense.
From my personal experience (more anecdotal evidence), I’ve seen Conex boxes in the desert storing U.S./NATO ammo, and have personally used ammo stored in hot humid conditions for long-term with no problems. I’m talking shot shells (some old enough to be some sort of cardboard or thick paper rather than plastic), .22S/L/LR, various old surplus calibers (some with slight corrosion), etc. not in ammo boxes or with AC during the summer, for decades.
I’m leaning towards heat doesn’t matter as long as humidity is controlled.
What say you?