If that barrel is badly rusted, it might be ruined. So you have little to loose. First thing I would do is take some solvent and a good brush and scrub that bore with a strong solvent. Then I would start pushing solvent patches through it.

Don't be surprised it the patches keep coming out with a rust color on them. After you have solvent attacked the bore good. Run dry patches through it. Even if they come out with rust traces.

After you have the bore dry, try shooting it with 80 grains of Pyrodex RS and some sabots and see how well it groups. If it shoots good, then there is some things you can do to the barrel to protect it.

If the barrel will not shoot. Get some maxi ball. And then get some NAPA Auto Parts valve grinding compound. Use the valve grinding compound as a lube and shoot about 10 of them maxiball through the barrel. Some people use JB Bore Paste because it is not so abrasive. You have technically fire lapped that barrel. Then clean the rifle real good and see what the patches look like.

After you have it cleaned, take it back out and shoot it with a sabot or some other projectile. See how it shoots. If it shoots good, then it is time to protect the bore.

If not, you might have to decide what you want to do with the rifle. You could try lapping it more. It might ruin the bore it might not. You really have nothing to loose at this point. You could call Knight also and see what they recommend.

If you get the barrel to where it is shooting, you want to oil that bore REAL HEAVY. Do not be scaredto put it on real thick. I would use some Break Free CLP. Oil it well and a few days later patch the oil out and check it again. Then re-oil it. Eventually the patches will come out gray. While that is not rust anymore, you still have a damaged bore. But if the rifle is a shooter, consider it a lesson learned.

I hate it when someone gives me a rifle they did not care for and then expect me to get it shooting again. I was given a rifle in terrible shape. I lapped the bore, and scrubbed it. To this day, I can not get a white clean patch to come out of it. But the bore is oiled and no rust color comes out of it. The strange thing is, I lowered the powder charge down to 70 grains and it is a good shooter.