I suspect that "магистр гуманитарных наук" is only used to translate the English word.
Actually, there is no M.A./M.Sc. division in Russia, so if you need a generic term, it is simply "магистр". If you need the full title, however, you will have to check the exact major, like "магистр истории", "магистр литературоведения", "магистр востоковедения и африканистики".
All such titles have been introduced comparatively recently, and there is no common way to abbreviate.