Why an autoloader should not store heavy shells horizontally

Could an autoloader store its shells horizontally?

Not safely at sea; heavy shells are better stored vertically and secured so they cannot roll with the ship's motion.

A viewer proposes storing shells horizontally for easier mechanical access. Heavy rounds could roll as the ship moves, making the arrangement unsafe at sea. Naval ammunition is instead kept vertical, stacked, and secured against motion. Automating shell movement may even be easier than safely managing the separate propellant charges.