I borrowed my father in laws Remington 7600 .270 win to use a few months ago. I have shot it less that 10 times. I cleaned it and oiled it before i put it back in the case.

Today I opened it up and there was rust all over in spots on the barrell. I dont know why this would have happened. It is a black barrell.

Will this come off and look like it did before I borrowed it, How do I get this rust off?

Thanks