Free: Contests & Raffles.
how much do you want to spend?so many answers without a price range to work with.
why do you want a 20 gauge?
nothing wrong with a 20, as long as you can shoot don't know much about autos, so i'll recommend the old standby 870 wingmaster. should be able to pick up a used one fairly cheap, they weigh about a pound less than the 12.
Get a nice light 20 gauge with a 26 inch barrel and choke tubes....one that fits well and most important one you like. Open the choke up....most shots are 15-30 yards. Less pellets in the 20 with an open choke means less risk of have your target rendered worthless as far as table fare appeal is concerned.....and more importantly, makes you a better shot.