Nope, not true. First, most 20 ga barrels fit a 20 ga frame, not the bigger 12 ga frame. There are some 20 ga barrels that fit a 12 ga frame, like those that are part of a four barrel set, but you hardly ever find these. Lastly, it would be shear luck for another barrel to fit your frame without a gunsmith fitting it. Oh, by the way, don't forget about the handguard. This can be a problem in its own right with 2 screw vs. 3 screw forend iron, plus the location of the mounting stud on the barrel.