With your military experience you may wish to watch the GunsAmerica tactical rifles section or Gunbroker.com. You may come up on a tackdriver for a very good price.

In my experience it isn't hard to find a decently priced 300 yard rifle, but a 400+ yard rifle is a different creature entirely.I have only had few rifles that shot as well as custom jobs stock out of the box--a 7mm Mag Sendero for example. The other low cost surprise was an 11 pound 308 heavy barreled Savage varmint rifle ordered with an accutrigger.My rifle remains a 0.5 MOA rifle out of the box. Thats impressive for the relatively low cost!

Whatever you choose get great optics and have fun!!