maverick 88?? its made by mossberg and stuff intercahnges with the mossberg 500. ive owned one for 10 years and it only jammed on me once (a good cleaning fixed it).
im not sure if them come in 20ga tho. check walmart, they can order almost any gun.
http://www.maverickarms.com/
EDIT- nevermind you said semi auto all the mavericks are pump, ignore my post haha