With 500 yards as your limit for the time being, I would avoid the efficient 6mm's that most like for target shooting. Wind effects are pretty minimal at this distance, and if you're training, you'll want to get the most out of the distance you got. It may be counter-intuitive, but shooting a round that has little recoil and bucks wind easily won't teach you much.
A short barrel 308 would be a good choice since factory ammo is cheap and good, barrel life is great, and the lower velocities will help exaggerate wind conditions. Recoil on a 308 is about perfect in my mind since you still need to manage it, but it's not anything crazy. I've got a 20" AAC-SD that shoots 1/2 MOA groups with factory 175gr FGMM, and I run it out to 1,000 yards without problems. For 500 yards, you might consider the 16.5" barrel version.