I believe such a conversion barrel exists, but I am relying on Google Fu for that answer. I have no personal experience with such a conversion. You are correct they are the same size gun. I do know the .40 to 9mm conversion barrels work fine.Hey vert can you just drop a glock 20 10mm barrel into a glock 21 and have a 10mm or is that not possible. They look identical except for caliber.