Here's the Musket I'm thinking of. 1643 Matchlock Musket.
Could a Matchlock musket explode as the soldier was taking aim? (Holding it close to his face.) What could cause it to explode? And what kind of injuries would result from it?
I'm writing a book that has a former who's Musket exploded as he was aiming. He didn't die, but had gruesome injuries. It caused him to lose most of one hand, an eye, an ear, and half his face is mangled. I'd like to know if it's accurate.
I looked up "gun explosion injuries" and the images showed bloody faces and mangled hands, but those were for modern guns.
Sure it could explode. A muzzle-loading gun has to be loaded one step at a time, like the demonstrator is doing in your linked video. Mix up the steps , and it won't work. And in the chaos and excitement and terror of a battle, it was not hard to mix up the steps. That's why soldiers were drilled, over and over, so that the loading method became automatic.
There are various ways an explosion could happen. Because it was not obvious if the gun was already loaded ( he had to look at how far the ramrod was sticking out of the muzzle and might not have marked it for that) , and because an excited soldier might not notice his gun had fired, it was possible to reload on top of an unfired charge. One sampling of guns left behind on a Civil War battlefield showed such double-charges, one with many charges. If repeated flashing of the priming powder finally set off the first charge, that would make a pressure spike...and if one or two of the super-imposed charges went off, that could blow the barrel. I can imagine a simple mistake: grabbing the ball between the teeth, tearing off the rest of the cartridge with the powder, then ramming the paper-wrapped powder down the barrel instead of pouring out the powder first. The paper blocks the touchhole inside, preventing the first flash from reaching the charge. The soldier reloads, and the second flash finally burns through the paper and sets off the first charge, which collides with and sets off the second.
There are other ways, too: the soldier gets angry and decides to load three balls on top of three charges of powder. He leaves the plug bayonet in the end of the gun, or he loads his musket then drops the muzzle into the mud and plugs the barrel ( though, these two you could expect to swell the muzzle, maybe not burst the breech). Perhaps this is before musket barrels are well-proofed, tested, and his has a badly-welded barrel. Or his commanding officer has taken the money given to him for equipping his solders with good muskets, bought very bad old ones and pocketed the difference ( not uncommon), and the soldier has one that's damaged.