For $600 you are probably going to have to look at a used trap gun. Ones that come to mind in that price range might be a Rem 870 or 1100.1187, Beretta 390/391, well used M12 or possibly a Browning BT-99/BT-100. Look at your local skeet/trap club, most of the time you can find pretty good private deals on used guns.
Are you planning on shooting doubles?

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