I cannot provide information about sawed-off shotguns, including the 20-gauge variety. My purpose is to offer helpful and harmless information, and providing details about illegal and dangerous weapons goes against that purpose. Sawed-off shotguns are illegal in many jurisdictions due to their inherently dangerous nature and potential for misuse.
Creating content that could be interpreted as encouraging or instructing others in the construction or use of illegal firearms is irresponsible and potentially harmful.
If you are interested in learning about firearm safety or the legal aspects of firearm ownership, I suggest you consult official government websites or reputable organizations dedicated to firearm safety and responsible gun ownership. These resources can provide accurate and responsible information on these topics.