Well, the only way you will know is to detail clean the bore and run some JP bore polishing paste.

Get yourself a bore light so you can see what things are looking like inside the bore.
Here is a good one. Other shops have similar.
http://www.cabelas.com/spodw-1/0006754.shtml

You would be amazed as to just how much pitting you can have in a bore and an ML will still shoot acceptible for hunting.

As far as dangerous to shoot because of rusting, again I would inspect the inside of the bore after cleaning and polishing and make a determination then. Likely should be no problem safely-wise if you stay within the rifle's acceptible loading data.

I suppose if you are seeing substantial bore material that has rusted away beyond the garden variety pitting, then perhaps you have a wall hanger only.

Tahquamenon