I understand the OPs stipulation but this is all kinda crazy... if I had the $$ for it no doubt Id consider a SCAR in 7.62 NATO... but then Id probably opt for a high end "AR10" instead, ie Daniel Defense or Barret...
Given you dont have the $$ the best thing for a quality maintable rifle is going to be a DPMS pattern big bore AR (not the brand just saying the pattern)... if you buy them during one of the frequent 15% sales, an Aero M5 runs about 1k before optics..
The big ARs may look similar but they are way different to shoot and like small ARs you have lots of growing room and support with different caliber and applications in uppers (ie a 16 inch .308 for blasting with a red dot and a 22 inch 6.5 creed with a scope for the long game for instance, stocks, high end barrels, adjustable gas blocks, buffer springs, triggers, etc and easily available reasonably priced magpul mags)..
It is the most practical thing - big bore ARs just make sense.