Don't forget you will have to weld up the extractor rims to match the 20 bore chambers . I have done this kind of work and on some guns it is easier to weld up the old rims first so as you do not need to bore out to an over large diameter .There can also be a problems refitting the ribs to the new smaller tubes and with the forend loop dropping down lower ,dependent on the type of forend latch so you may want to consider a new one .