Barrack or barracks? [duplicate]
Solution 1:
I haven't been in the army, but as a born-and-bred American, I don't think I've ever heard "barrack" used in the singular either; even in trying to keep to the tone of your mother's story I'd go with the plural "barracks." It's just one of those language idioms that doesn't translate (well) directly.
It's possible the Brits use the singular "barrack", but you'd have to ask one of them.