Temperature fluctuation significantly impacts ammunition and firearm reliability. I know they wont blow my gun up if it gets inordinately hot. Not because they're tactical.because, as reduced recoil rounds, they're lower pressure than standard. 357 magnum that I know no matter how hot it gets, they can't damage the gun (or me.)ДЇor a long-gun, I only keep a 12g pump locked away, but I keep it loaded with reduced recoil tactical loads. They're still plenty powerful out of a standard length barrel, and they're so much lower in pressure than. 357 Magnum revolver, but I keep it loaded with. But at least they can't blow up your gun.ДЇor these reasons, in my truck, I run two things only: I keep a loaded standard size. (For clarity's sake, a non-temp-stable powder does not mean it's going to spontaneously go off, you'd have to burn it in a fire for that to happen, but it will produce higher pressures when fired)Д¬old temps have the opposite effect, decreasing overall pressure and reducing muzzle velocity (and trajectory, and power). The only MILSURP ammo I would trust is direct from Lake City, and only new-production, not stockpiled stuff from before 2013 - the older stuff has a non-stable powder type. There are no temp-stable pistol powders, they are ALL double-base and contain nitrocellulose, and no way to know what powder your store-bought rifle rounds use. There are some rifle powders that are all but immune to this, most of which were developed either in Australia (cuz.outback), or by request of the US Army (cuz Iraq/Kuwait/hilarious 120 degree temps that result in a blister on the tip of your nose when you try to qualify in-country and use the nose-to-the-charging-handle method, ask me how I know). The temperature in a locked car, in the summer, in direct sunlight and hot temps (such as the OP's California) can approach 140 degrees. It can increase it dangerously, particularly in pistols. High temperatures increase gunpowder combustion speed, which increases pressure. I never see anyone talk about it when this topic comes up.

There's a lot more that goes into storage method and firearm choice for vehicle use than most people realize. I've been carrying concealed for years and have plenty experience keeping weapons in vehicles long-term.

Center console gun safe