Royal ordnance factory is a term that refers to the government-owned factories responsible for producing arms, ammunition, and explosives. A few synonyms that can be used to describe these factories are military arsenal, munitions factory, armament works, and defense plant. The phrase 'cannon foundry' can also be used to describe these factories that specialize in casting cannons. The term 'armament manufacturing unit' can be used to describe factories that produce weapons for military use. These synonyms help to convey the same meaning as royal ordnance factory but allow for more variety and precision in language use.