They started plating pennies in 1982.a copper penny like from the 50's or earlier because they are full copper and it won't scratch the bluing but will remove the rust spot.
+2 on the Evaporust. I've used it inside 2 muzzleloader barrels with good results. Also used it on a box of badly rusted molds given to me for safe keeping. A film of Evaporust was left on the molds; fours months now and no rust. Vinegar sounds good. Haven't tried but its way cheaper. Don't really need to scrub with Evaporust. Downside is its expensive and won't last forever.
View attachment 1172856
I let the rust in the barrel soak in Kroil for a week or so, then go at it with JB Bore Paste. It has worked fine for me.You can use some steel wool on a jag to get the worse of it out of there. Then, you could use something like JB Bore Paste to try to clean things up a little bit more.
Even with the rust it would probably shoot just fine--it will just foul faster and be harder to clean.
If the rust is on the exterior of the barrel I would not use 0000 steel wool. That's abrasive and you'll lose some of the finish. Use a nickel (5 cent piece) and coconut oil. It'll remove the rust but leave your finish intact.
Inside the bore, good idea to remove the rust but you don't want to harm the lands. There might be pitting but as mentioned earlier, even a pitted barrel can still shoot accurately.
I have used 4-0 steel wool on small rusty spots on blued barrels for over 50 years. The steel wool may have removed some finish but I can't tell that it has. BTW, I still have these guns and they function well and look OK to me.If the rust is on the exterior of the barrel I would not use 0000 steel wool. That's abrasive and you'll lose some of the finish.
I thought it was earlier like 68 or so, nowadays my brain is not what it used to be.They started plating pennies in 1982.