If you have never seen a "cook off" - then I hope you never do. I have seen it happen once while shooting NSSA - Civil War Muskets - there seems to be a misconception of the use of paper cartridges in muskets. Yes - early smoothbores, the tail was torn off and the powder dumped down followed by paper and ball. If you look at drill manuals, for flintlocks, the pan was primed from the cartridge first, the frizzen closed, and then the barrel loaded. One post referenced artillery - what you are talking about is why the vent is ALWAYS "thumbed - i.e., the vent is covered tightly with the number 1 man's thumb covered by a "thumbstall". If you have ev er done this position on a full size cannon, you know how hot the gasses are being pushed up against the leather of the thumbstall as the bore is sponged between rounds. It is also thumbed during the loading. Think of the bore as being the same as a "tobacco smoking pipe". As your mouth draws on the stem, air is drawn into the bowl causing the tobacco to burn from the already present hot ember of the lighted tobacco. The same can happen in a muzzleloading firearm - flint or percussion. In theory, if a very minute ember were present in the breech - hot fouling as an example - and it is not hot enough to ignite the powder that has been poured in - as you push a tightly patched ball down the bore you are pushing gas (air) down the bore and compressing it - it leaves through the vent or nipple and that action could cause the ember to get hotter and ignite the powder. As the air is being pushed down, the it is being pushed through the granules of powder, possibly over a cool ember that with the introduction of air causes it to get hotter. I've shot BP for over 45 years - military muskets, fowlers, flint and percussion. The only time I've seen it is as I said, in a military musket and the reason that it happened was probably due to the rapid rate of fire during a timed event. A rifled musket with a mine ball can be loaded and fired 3 times a minute. Your typical round ball - flint or percussion - is not going to be fired that rapidly. Everybody does it differently for round ball. When I'm shooting, I always spit patch the bore between shots - first to remove some of the fouling and second to take care of any hot spots. If a percussion or flint rifle is not vented correctly - i.e. the drum or the vent is placed too far forward of the end of the breech plug, fouling can build up on the face of the breech plug and possibly cause an ember problem. Going back to muzzle loading artillery, premature ignigion was a real problem when the artillery was loaded quickly during battle without sponging between shots. You NEVER wrap your hand around a cannon rammer for that reason. There were a lot of "one armed" artillerymen as a result.