Well you have a good rifle. You might have your distance expectations too high. 200 yards can be done. But 300 might be a little far. Maybe with lots of practice and good optics you can get it done. But I personally would not try it.

I would shoot Black Horn 209 powder and 250 grain Shockwaves. Start with 90 grains of loose and start working up and see when the accuracy falls off. You have a good gun. Enjoy it.