Guys: I own a gun I bought from a man who had it in a gunsafe when his entire log house burned down around the safe. The only damage was that the wood grips burned off, and the fire department apparently got the door to the safe open, and got water on the gun, as half the springs had lost their temper, and were rusted when I bought the gun for $35.00. I bought new springs from the factory, had the finish re done, and put custom made grips on it. The gun shoots as fine as I can hold it.
My concern when thinking about putting the powder into an old metal framed cooler is from falling debris during a fire. The cooler can keep the powder cool even during a pretty good fire, and protect it from falling debris, too. I have no illusions that a Coleman ice chest is going to contain burning black powder in any meaningful way- certainly not enough to cause an explosion that would not have occurred otherwise! Now, Ammo cans are much better constructed, and much tighter sealed. I would worry about keeping powder in one of those. That is why I did not mention them for storage, although we probably all know someone who stores his powder in such containers. Not me. Like you both, I have full faith in the containers the factory puts the powder in to ship it across country! Where we differ is that I want there to be a chance for the fire department to save some of my possession from burning up, by spraying the area with water, but still protect the powder in its containers from the adverse affects of the water. By buring the powder in the box in the ground, if there is an explosion-- a big IF -- its going to explode up, where it will do little harm, rather than sideways where people can be hurt or killed. I too have tried to set off a pound of powder by shooting the can, without success. Yeah, it was a surprise to all of us, too.