Too much of the gun would have to be replaced to "convert" it. If you want a 20 gauge pump shotgun, you'll need to just buy a 20 gauge pump shotgun.
Big Dawg No. 1431
Carolina Glocker No. 1431
"Freedom is a system based on courage" (Charles Peguy)
"Know where the attack against you is likely to come, whether on the street or in court, and have a proven counter already in place" (Mas Ayoob)