its not as easy to chamber just a brass case from a mag regardless if fired from your rifle.....they will chamber if laid on top of the mag though.
to chamber from the magazine they really need to be fully made up.
if you buy once fired from some one else and you want to check if it chambers in your rifle when you resized it then just lay on top and close bolt.
i have done just a brass case from the mag but have to guide it in and slowly chamber