I've managed to record some slow motion footage (it's not that slow, but still impressive) of some muskets firing off.

https://youtu.be/XVwMYh7qhcQ

https://youtu.be/1IJmUbF56Vc

https://youtu.be/efizxIsgZPw

(I realised I can't embed all of them directly, so you'll have to follow the above links.)