Tell that to the US military storing millions of rounds of 5.56 in fat 50's in the 120°F desert in the middle east for the last few years...they may need your help...
Military HumVees and other vehicles have to operate reliably in 120 deg
F heat so we can agree that the temperatures under the hood will be even hotter than ambient air temps. A good range to look at is what automotive electronics have to withstand which is -40C to 85
C. Underhood temps exceed that range so ECUs and other modules are typically placed in the passenger compartment. One source of information is from the SAE
From SAE J1211 Handbook for Robustness Validation of Automotive Electrical/Electronic Modules:
TABLE 1—RECOMMENDED TEMPERATURE EXTREMES
Underhood
Location Maximum Temperature
-- Dash Panel 140 °C ( 285 °F)
-- Engine (Typical) 150 °C ( 300 °F)
-- Choke Housing 205 °C ( 400 °F)
-- Starter Cable Near Manifold 205 °C ( 400 °F)
-- Exhaust Manifold 650 °C (1200 °F)
I can imagine the worst point is shutting the engine off in hot weather and there is a long heat soak before the compartment cools off. From a ammunition reloading company "As a rule of thumb, any temperature over that which is comfortable to a person is accelerating the decomposition of smokeless propellants."