There are ancient ships thousands of years old in service to the Imperium. Their large cannons no longer have automatic reloading systems and nobody wants to risk breaking anything by trying to fix it, so they use large press gangs to manually load these ginormous guns. Some of these people have lived in the bowels of these ships for generations and develop their own cultures around these weapons, never knowing of life outside the depths of the ship. They will wage war against the other gun crews, unaware of their true purpose.

The rest of the ship's crew often leaves these loaders forgotten in the depths of the ship. They care not how the guns are loaded, only that they fire. To be alive in such times is to live in the cruelest and bloodiest regime imaginable. A ship's crew is no different.

Then they'd paint it red because red makes everything go faster.... Literally