If it is very light surface rust, see if you can get a good size rag threaded flat between the barrel and tube. If you can, soak it with some solvent and with the gun held between your knees, legs, whatever way you can run the rag briskly like you would shine a shoe. First against barrel then tube. You can exert all the pressure on the barrel you want; go easy on the tube so as not to bend it.
Make sure you get into the corners at joining areas front and rear. Repeat with a clean rag nas needed.
Clean and dry, coat with protectant with small piece or clean rag. With it being a protected area you leave a very very thin film of protectant on this area; it is naturally sheltered when you wipe down the exterior of barrel and tube.
I would also recommend Tetra Gun Grease as a protectant. It can be smeared/wiped into the metal/finish. Wipe dry, them buff with clean rag. It leaves a dry film that is not only a decent lube, it also protects well well against rust.
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