My first impression was the powder looked like large chunks of dog food. They must need a very slow burning powder to accelerate that heavy of projectile down a long barrel without over pressuring the barrel.
my son brought home a fired 5" case from a destroyer he had a visit on during ROTC. darn kid carried it around on his sea bag for a week!.
they now use steel cases and there is a long steel tube (maybe 3/8" dia) that starts on top of the primer and runs maybe 2/3 the length of the casing. i believe it is filled with a fine powder of some sort that the primer ignites and then that ignites the main charge