In situations like this (which I consider frustrating and desperate), I have resorted to much more aggressive tactics and I have been almost always pleased with the results.
I respect the opinions of those who say that you shouldn't use stainless brushes in barrels. My only qualification to that statement is that I wouldn't use stainless brushes in match quality barrels with a high level of smoothness and uniformity. In my opinion, the rules are totally different when it comes to old pitted military barrels. In those situations, you actually WANT to remove some metal from the barrel because the roughness and pits are acting like a grater as your bullet passes over, and this causes insane fouling, pressure spikes and poor accuracy.
Here is a thread that I started in 2008, and it includes photos to show the results of what I did, as a last resort, to one of my barrels. The results speak for themselves:
http://www.canadiangunnutz.com/foru...1-Rust-Valve-Grinding-Compound-and-Steel-Wool
Since that time, I have done similar things with other barrels. In fact, I even went out to Lee Valley and bought some of their fine and superfine lapping grit and made my own paste up by mixing it with grease until it resembled grey peanut butter. You should see what that Lee Valley stuff did to an old military Mauser .30-06 whose barrel was so rough and fouled, I thought it was a smoothbore when I got it. (The rifle now consistently shoots 3/4 m.o.a. out to at least 600 metres.)
From what you have described, your barrel needs internal polishing to remove the baked-on crud and then to smooth the rough metal down. I wouldn't worry about opening up your bore diameter a bit because your rifle is so inaccurate now as to be unusable; therefore, nothing ventured, nothing gained. Those Ross barrels had a reputation for tightness, anyway, so a bit of internal lapping shouldn't hurt too much.
At worst, you'll have to buy another barrel. But I would always try the aggressive treatment before giving up.