[CMichael]

I have left several boxes of shells in my trunk for several weeks. I has been quite cold.

Are they still okay to use? Did the cold do something to the metal?

[goosegunner]

They are just fine.
Cold don't make permanent changes in metal, and I think the same goes for gunpowder. However if the shells still are cold (colder than -20 celsius) when you (try to) fire them, some gunpowders don't work well or don't work at all. I have had most problems with extremly cheap shells and (Federal) magnum shells. the most usual problem is trap-loads that have so low pressure at extreme tempratures that they don't cycle semi-autos.

If you wan't to shoot in cold weather, the best thing to do is to buy a lot of different shells, put them in the freezer and test fire them. Some of the loads that seems to work fine might have different velocity when they are cold and therefore make a different pattern. And be carefull if a shell don't fire, it may have got delayed ignition, so leave it in the chamber for a few seconds before ejecting it. (I have never had any problems with shells that have been cold and then warm again.)

[Dave McCracken]

Cold's not a factor, after they warm up a bit.

OTOH, high temps cause powder to degrade while sending pressures up. I don't recommend leaving ammo in cars in the summer for extended periods of time.

[MonkeyMan]

Cold where you are won't be a problem. The History channel the other day had a show about "The Lost Squadron" in Greenland. Those guys dug out a 20mm cannon from a P-38 Lightning and fired it after it had been buried in several hundred feet of snow and ice for more than 50 years. Made a real mess out of a 55 gallon drum.

[9mmMike]

Cold shells will crack. Probably not a problem while shooting but forget about reload possibilities. The plastic becomes quite brittle when chilled to extreme degrees. Pun intended.
Mike

[CMichael]

Thank you to all who responsed :)