300 Blackout bullets. They're the right weight and you can choose between high BC or hollowpoints that expand at the expected velocities, either way you'll get better BC than traditional flat-nosed 30-30 bullets. 300 is typically a bit slower than 30-30 at the muzzle with the same weights, but at distances the blackout bullets will still expand well at the reduced velocity. Just don't use in a tubular magazine.
If this is for hunting, I'd pick 110 grain TTSX (the 300 BLK version). It should do well out to 300 meters in terms of flat trajectory, energy and expansion.
500m with 30-30 and open sights is probably not good for hunting. For targets, you could try one of the higher BC match type bullets. I don't have any specifics.