Ammunition will not spontaneously fire in a hot car under normal conditions. The primer compounds in modern cartridges need temperatures of at least 250°F to auto-ignite, and the propellant powder requires even higher heat, around 300°F. A car parked in direct summer sun typically reaches dashboard temperatures of about 160°F and cabin temperatures around 116°F, well below those thresholds. That said, heat is still bad for your ammo, just not in the way most people fear.
How Hot a Car Actually Gets
Arizona State University measured temperatures inside parked cars during summer heat and found that vehicles sitting in direct sun reached an average cabin temperature of 116°F within one hour. Dashboards, the hottest surface in the vehicle, averaged 157°F. Steering wheels hit 127°F, and seats reached 123°F. Cars parked in the shade stayed closer to 100°F inside, with dashboards around 118°F.
Trunks tend to run slightly cooler than dashboards but hotter than the cabin overall, especially in dark-colored vehicles. Even in the most extreme desert conditions, no part of a passenger vehicle reaches the temperatures needed to set off a round.
What Temperature Ammunition Actually Needs to Fire
Modern primers use a compound called lead styphnate. According to U.S. Naval ordnance testing, cartridges using this compound have cook-off temperatures between 250°F and 330°F, meaning they can sit at those temperatures for an hour without auto-igniting. Below that range, nothing happens. The propellant powder inside the cartridge case is even more heat-resistant. Testing on 7.62mm cartridges found that the propellant didn’t cook off until chamber temperatures reached roughly 304°F (151°C), and even then, the round had to be enclosed in a simulated gun barrel with sustained, direct heat transfer for several minutes.
The gap between a 160°F dashboard and a 250°F primer ignition point is roughly 90 degrees. That margin means a round sitting on your dashboard on the hottest day of the year in Phoenix is still nowhere close to going off.
The Real Risk: Heat Damage Over Time
While your ammo won’t explode, repeated heat exposure quietly degrades it. The main component of smokeless powder is nitrocellulose, a compound that is inherently unstable and slowly breaks down through chemical reactions. Heat accelerates this process significantly. As nitrocellulose degrades, it produces nitrogen oxides and acids that act as catalysts, speeding up further decomposition in a self-reinforcing cycle.
Manufacturers add chemical stabilizers to slow this breakdown, but those stabilizers get consumed over time. Once they’re depleted, degradation accelerates rapidly. The practical result is ammunition that becomes unreliable. The Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI) notes that extended exposure to high temperatures may cause lower pressures, incomplete powder burn, or failure to fire. In some cases, degradation can actually increase pressures, which raises the risk of a dangerous overpressure event when you eventually fire the round in a gun.
This doesn’t happen from a single afternoon in the car. It’s the cumulative effect of leaving a box of ammo in your truck all summer, day after day, cycling between extreme heat and cooler nights. That repeated thermal stress wears down the stabilizers faster than cool, steady storage would.
What Happens if Ammo Burns Outside a Gun
Even in the unlikely scenario where a round is exposed to open flame (a vehicle fire, for instance), ammunition outside of a firearm chamber behaves very differently than a fired bullet. Without the sealed steel chamber of a gun barrel to contain pressure and direct it, the cartridge case simply ruptures. The brass or steel casing is the weakest structural point, so it splits open and the bullet moves very little. Air Force Research Laboratory testing on ammunition in fire scenarios found that loose rounds deflagrate (burn rapidly) rather than detonate, and the fragments don’t carry the velocity or energy of a properly fired bullet. It’s still dangerous to be nearby, similar to small firecrackers going off, but it’s not the same as being shot.
Storing Ammo in Your Vehicle
SAAMI explicitly advises against leaving ammunition inside a vehicle or trunk on a hot day. If you need to transport ammo during summer, a few steps help preserve its reliability. Keep it in the original packaging or a sealed ammo can, which provides some insulation and blocks direct sunlight. Place it on the floor of the cabin rather than the dashboard or rear window shelf, since lower positions stay cooler. For longer trips or extreme heat, an insulated cooler with ice packs keeps temperatures stable.
For day-to-day storage, bring your ammunition inside when you get home. A closet shelf in an air-conditioned house is close to ideal. Cool, dry, and consistent temperatures are what keep powder stable and primers reliable for years. Ammunition stored properly in a climate-controlled environment can last decades. The same ammo left in a hot vehicle through repeated summers may become unreliable in a fraction of that time, and unreliable ammunition is arguably more dangerous than no ammunition at all.