With a budget of $500, I would forget the semi-auto and look for a good-condition used pump shotgun. Cheap semi-autos lack the reliability and ruggedness you demand for self-defense or tough use hunting. A Remington 870 would be a good choice in a pump shotgun. Mossberg, Benelli, Browning, Ithaca 37, and Winchester M12 would all be solid choices as well. The Remington and Mossberg will likely be the most common on the used gun racks. Forget the 3.5", no need for that. Look for a good used turkey gun. A Remington 870 or Mossberg 500 designed for turkeys will have a short barrel of 24" or less, usually around 21", have sling swivel attachments, and take screw in choke tubes. Perfection. Barrels for the 870 or 500 are readily available on the market so if you find an suitable gun but it has a 28" barrel, you can buy a shorter turkey barrel and keep the longer barrel for other hunting uses, or have the long barrel cut and threaded for choke tubes.