Stevens/Savage Arms 94c or 940

[ironsniper1]

hey all my grandfather has this single shot 12 gauge shot gun, pretty much all the bluing is gone and its got rust from where my uncle did not store it correctly, but i have it for now and plan to fire it soon but how do i go about removing the rust without damaging it further?

 

also my uncle said it had an issues of i believe popping open after being fire or something like that, should i just take it to a smith and have them look at it? and has anyone here ever fired one and is it 94C or 940?