While doing exercises in English grammar/vocab/whatever-it-is, I encountered a problem with the words "barrack" and "barracks". The task was to put all the NPs possible into singular without changing the meaning of the NPs (like: I can transform "the drivers" into "the driver", but I can't transform "Middle Ages").
The problematic sentences were:
1) The soldiers left their arms in the barracks. --- The soldier left his arms in the barrack(s). (well, I'm totally unsure about that one)
2) Barracks are buildings used as military headquarters. - A barrack is a building used as military headquarters.
Are my transformations okay? I can't find the explanation anywhere, I hope you'll help me! Thanks in advance!