can ammunition explode in a hot car

[DelLukas]

No, it is extremely unlikely for modern small arms ammunition to explode in a hot car. While high temperatures are a serious concern, a spontaneous explosion is not the primary risk. The real danger lies in the ammunition's components being damaged, which can make it unsafe to use.

Modern ammunition is designed with stability in mind. The propellant inside (modern smokeless powder) requires both intense heat and significant pressure confinement to detonate. A car's interior, even on a scorching day, typically won't exceed 170°F (77°C), which is far below the kind of heat needed to initiate an explosion. However, sustained heat can degrade the ammunition over time. It can cause the powder to break down, potentially leading to inconsistent burning rates when fired. More critically, heat can destabilize the primer compound, increasing the risk of a squib load (a bullet stuck in the barrel) or, in a worst-case scenario, a cook-off if the round is already in a hot firearm chamber.

The following data illustrates the temperatures involved and the safety margins built into ammunition storage standards.

Factor	Typical Range/Threshold	Notes & Sources
Car Interior Temp (Sunny Day)	130°F - 170°F (54°C - 77°C)	NHTSA & AAA studies on vehicle heat retention.
SAAMI Max Storage Temp	150°F (65.5°C)	Sporting Arms and Ammunition Manufacturers' Institute recommended limit for long-term stability.
Smokeless Powder Ignition Point	~300°F - 400°F (149°C - 204°C)	Requires sustained heat at this level in a confined space.
Bullet Cartridge Melting Point	Varies; brass ~1700°F (927°C)	The cartridge case itself won't melt in a car fire scenario.
Ammunition in Direct Flame	Can "cook-off"	Rounds exposed to direct fire can detonate sequentially, not as a mass explosion.

The key takeaway is to prioritize safety. Don't store ammunition in your car long-term. If you must transport it, keep it in a cool, shaded part of the vehicle, like the floor inside a case—not on the dashboard or seats. Heat damage is cumulative and invisible, so if you suspect your ammo has been severely overheated, it's safest to dispose of it properly rather than risk using it.

[SanLily]

I've been a competitive shooter for years, and this is a common clubhouse topic. Look, your ammo isn't just going to blow up. I've left ammo cans in my truck bed in Arizona summer. The real worry is it gets ruined. The powder cooks, and then your rounds might not fire right—low power, or a squib that jams your gun. That’s the actual hazard, not a Hollywood-style explosion. Just don't make a habit of baking your gear.

[DeBenjamin]

From a safety perspective, the risk of a mass explosion is minimal. However, consistently exposing ammunition to high heat violates manufacturer storage guidelines. This can lead to pressure inconsistencies, which are a firearm safety hazard. The prudent action is to avoid long-term vehicle storage. If ammunition is subjected to extreme heat, consider it potentially compromised and consult a professional about its safe disposal. Proper storage is a key component of responsible ownership.

[TravisLynn]

Think of it like leaving a soda can in the car. It doesn't explode, but it sure gets messed up. Your ammo is the same. The heat makes the chemicals inside weak and unreliable. You might pull the trigger and nothing happens, or worse, the bullet gets stuck. That can really damage your gun and is dangerous. So, just bring your ammo inside with you when you get home. It’s not worth the risk, even if it won’t blow up your car.

[AubreyRose]

The science behind this is pretty clear. Modern smokeless powder is chemically stable and requires a precise combination of heat and pressure to deflagrate (burn rapidly). A hot car provides heat but not the necessary pressure confinement. The cartridge case acts as a pressure vessel, and without a firing pin striking the primer to create a spark, the system lacks an ignition source. The energy required for a detonation simply cannot be achieved through ambient temperature alone. The focus should be on preventing degradation of performance, not on a highly improbable catastrophic event.