Once you cut that barrel, its cut forever, Plus you have the money for cutting and recrowning the barrel. Thompson makes a 20 inch barrel. They come up used here and other places from time to time. If it were me, I'd buy a used 20 inch factory barrel. Try it out and sell the other if I liked the 20 inch better. If not the 20 inch barrels are easy to sell if you don't like it.