I recently watched a you tube video on the firing of naval guns. The turrets and magazines and elevators,etc.
Its not brass cased fixed ammo. The projectile is loaded, then "bags" (for lack of a better term) of propellant are loaded behind the projectile. The last thing loaded into the chamber is where the black powder comes in. Its a smaller bag of Black Powder used as an ignition charge to get everything else lit up.
FWIW, BP often gives remarkably uniform velocities through a chronograph.
A uniform ignition might be a good thing when you are shooting 20 miles.