I'd look at Beartooth Bullets for cast bullets.

If you use a round nose or LeveRevolution bullet you should be able to get flat enough trajectory to handle 250-300 yards. I shoot a 35 Remington that is even slower & fatter than a .30-30 and I can shoot point-blank out to 200 yards. I'm sighted 1.5" high at 100. If I had to, I'd feel comfortable shooting out to 250 if I had a good solid shooting position and little wind.

I'm loading Hornady #3060 170 gr flat points pushed by 30 gr. of 3031 ignited with CCI 200 primers. Easily 1 inch groups at 100 yards from a Marlin 336. My contender with a 14 inch barrel, I loaded Sierra #2020 125 gr hollow point flat nose bullets with 24 grains of 4627 and again CCI 200 primers. I have not loaded the hollow points for the Marlin yet. They are devastating at pistol volatilizes and distance, but I've been told that they are really good out of the carbine.