The Ruger scout rifle might be what you want. It is a general prupose rifle, but the scope mounting system and shorter barrel are not ideal for longer range work. You should be able to hit human size tagets at up to 300 yards though. It is far from a target rifle however.
By requring iron sights and a DBM you are eleminating most of the better guns. Drop those requirements and there are many, many good options in that price range. A DBM is more of a hassle than an advantage and iron sights can be added later to any gun you choose for less than the cost of a cheap scope and mounts.