Because, in the main rulebook, under the section for weapon batteries, it says.
"weapon batteries form the main armament for most warships, ensuring that much of their hull is pock-marked by gun ports and weapon housings. Each bettery consists of rank upon rank of weapons: plasma projectors, laser cannons, missile launchers, rail guns, fusion beamers, and graviton pulsars."
The very definition of weapon batteries presented in the blue book is that you've taken all the specific weapons from the 40k universe and diluted them into a generic terminology.