Very good advice from Bill. If you want to do this yourself, you need to be patient and don't rush it. I suggest you clean the barrel good. Then you might try buying some krol oil, or liquid wrench or other penetrant in a quart can. Pour off a few ounces into a seperate container and then place the barrel, choke down, in it and allow it to soak for a few days......tap the end of the barrel with a block of wood every few hours and then soak it some more. I've heard that freezing the barrel and then heating it gives the largest amout of contraction and expansion so you might think about sticking it in the freezer for awhile before heating it. A cold choke tube and warm barrel should provide the largest clearance between threads.