If it was my gun I would soak the joint where the tube passes thru the takedown assembly and thru the bottom and into where it screws into the frame with Kroil or the quality penetrant you prefer. Grab my dual range heat gun and warm that area up real good, accompanied with smartly tapping the end of the tube with a smooth plastic hammer. Repeating as necessary till it comes loose. All this with shotgun upside down in a large bench vise with leather padded jaws secured to a sturdy workbench. Anything less than this setup and application of solvents and heat, is an invitation for results to vary per individual application. If it was my gun that is.