Sure it's rust. Wd-40 is an ok penetrating oil, not the best. But its easy on finishes.
Get a really hot wet towel, put it on the outside of the barrel. Take a can of duster, turn it upside down. Spray a shot or two inide the barrel. Tube shrinks, while hot rag keeps barrel warm. Use a small tiny adjustable wrench on the choke wrench. Use a wooden 2 jaw clamp where the barrel has some mechanical advantage. Like the magazine tube hole. Can add a rag or two between the wooden clamp.
Differential heat works better without oil. oils transfer heat more. You can put a bit of paper towel in the barrel to keep the liquid duster from cooling the barrel directly.