I use them interchangeably (though maybe I shouldn't). Adding to the confusion is the fact that pre WWII they are universally called "arsenals" (Hanyang Arsenal, Xigu Arsenal, Tianjin Arsenal, Guangdong Arsenal, Kaifeng Arsenal etc.), whereas after Mao took over, they morphed into factories with simple numerical names (626 Factory /66\, 296 Factory (/26\, etc.)