WWII,
If the rifling is gone only at the muzzle, then the counterboring suggestion applies. If the rifling seems to be gone all the way down the bore, then you'll likely find it all magically reappears when the bore is subjected to a proper cleaning. Guns that have not been properly maintained can develop enough carbon and crud build-up to fill the grooves. Especially if they were fired with corrosively primed ammunition and not properly cleaned afterward.
Something that I find loosens carbon and rust well is
Gunzilla. Just be aware it can take awhile. If I have a heavy build up like yours, I'll let the bore stay wet with Gunzilla for a week between patching and bronze brushing to see if carbon or rust comes out. If so, I repeat and keep repeating until it stops coming out. It can take several weeks for the carbon to stop coming out.
If you are less patient and don't mind using a product that can hurt finishes, then get some
Slip 2000 Carbon Killer, plug the bore, fill it with the stuff and let it sit 15 minutes, drain the stuff back into its container (it's still good) and patch and brush it out. Repeat until you stop getting carbon coming out.
Once the carbon is gone, you'll want to flush the bore with boiling water to remove the Carbon Killer or with a solvent like denatured alcohol or Gun Scrubber to remove the Gunzilla. Then apply
KG-12 to remove the copper. I put a sloppy wet patch through and let it sit 15 minutes and repeat once. That usually gets it all. It doesn't turn blue or green—just gets darker orange brown—as it picks up copper, but its fast and its capacity for copper is enormous. If you are unsure of your results, you can patch out the KG-12 and apply a cleaner that changes color in the presence of copper just to see if you are done yet.
It may take more patches, but another product that will remove the copper and which will give a strong blue color change is
Boretech Eliminator. It also loosens carbon somewhat. It is what I use for general bore cleaning. Just be aware it will turn a patch blue by attacking a copper jag, so use a plastic jag with it unless you want to invest in one of their special alloy
Proof Positive jags.
This may all seem like a lot to go through, but the Rem Oil you are using will let you get some of the carbon the way a penetrating oil would, but it won't be a thorough job and it's not likely to do anything to copper jacket deposits.