First off, don’t try this at home. If you burn yourself or burn your house down or blow yourself up I’m not responsible. You have been warned!
Gunpowder is some interesting non-linear stuff. At first if you think of gunpowder you think that it’s going to blow up. The next thing is thinking about the cartoons and how it burns when you lay down a line of it and set a flame to it.
Both can’t be true. Right?
Well, the video above shows a fairly fast (Power Pistol with a bit of Winchester 231 mixed in) powder burning. It’s not blowing up — it is just burning.
But how does that explain how it propels a bullet down the barrel?
That’s the non-linear part of the mix. The way smokeless powder burns is related to the temperature and pressure it’s subjected to. At normal atmospheric pressure it’s fairly benign. When it’s sealed inside of a cartridge it’s not. Within microseconds of the primer setting off the powder, the pressure starts to rise precipitously. As the pressure and temperature rises, the powder burns faster, which causes the pressure to go up and so forth. Eventually the bullet starts to move down the barrel and the combustion chamber starts to get bigger (though the pressure keeps going up for a short while). Eventually the bullet travels far enough that the pressure finally starts to drop back down again (since it’s still higher than atmospheric pressure it’s still pushing the bullet out).
This is exactly why seating a bullet too deep can cause problems. Since the space inside the case is smaller (even if by a tiny amount) the pressure can build up ever higher and eventually go higher than the rated pressure of the cartridge and the chamber.
I’ll have some more posts on this later breaking down the process even more.
Next time I’ll bring along some graphs! :-)