Ruger Mark II

[Walker1]

I am new to this and really do not know what I am talking about. I have a Ruger M77 MarkII in 7mmRemMag. It was my first rifle that I bought when I was 17 yrs old. The casings started jambing every time I shot it. I took it to a friends gumsmith and he said that I had a lot of rust buildup. Tells me there is not much I can do about it, the rust will never go away, just keep it clean. He suggested that I bore it out. Can I do it to this rifle? What would be a good caliber to go with?