Not to be one to talk anyone out of a new rifle purchase, but a decent 5.56 AR with a 12.5" barrel or more, with decent ammo and optic can reach 600 very handily. Shooting 5.56 at distance will be a great learning tool for wind, while keeping recoil low. Obviously I wouldn't recommend a 12.5" AR as a "600 meter gun", it's definitely toward the end of useful range, but the important things that come out of learning that way, such as "fundamental" marksmanship principles, understanding ballistic trajectory, learning wind calling and effect, and the optic, all transfer to other rifle platforms.