i'd also like to add that over 300m the practical application of most people's shooting skills as it pertains to the ethical take of game is quite frankly, over-exaggeration and utter bullsh*t. unless you're hitting a half-MOA dot for your given range factoring in wind, elevation, barometric pressure, the coriolis effect, your ballistic coefficient, relative humidity and heat mirage with no dope chart and no spotter i say you're full of it and more likely to end up wounding an animal. at your stated 500m, if you were able to achieve groups consistent with the above then you'd have a 2.5" group. to achieve that kind of boring regularity you need four things: an exceptional weapon, an excellent cartridge, good glass, and a ton of trigger time.
if you have the skills to pay the bills then you wouldn't expect unrealistic results from calibers that cannot obtain them. 7.62x54R and 7.62x39 are great rounds, but accuracy out of nearly any platform that shoots them is going to be 1.5MOA at best, and that's at 100yds and in a good platform. 7.62x39 is not a good choice over 300m, and you'd be lucky to hit a 5" target with an exceptional platform with that round. after 300m the drop is significant as is velocity loss.
notable exceptions for MOA expectancy would be a Rifle Dynamics DMR build or an SVD. Both are very, very far out of your budget.
long story short: i think your expectations, your caliber choices, and your budget are entirely out of line with one another.
