Watching Mythbusters today they claimed that the first explosive was… gunpowder.
I have to break it to them but it doesn’t explode. It burns really quickly sure… but it’s not classified as an explosion.
It goes through a process called deflagration — which is a sub-sonic combustion. This is opposed to what happens in a explosion which is a supersonic, and quite violent, oxidation of the substance.
I’ve already shown a while back it that it burns quite slow (at least in ordinary atmospheric pressures) It takes more than a pressure wave to get gunpowder going… it takes pressure and heat.
If all it took was pressure or a shock wave… you’d be in some trouble since your magazine, just a tiny bit below the action (quite literally), would go off easily. It doesn’t. It’s quite safe, relatively speaking.