I think you have unrealistic expectations of cast boolits in rifles.
Flat trajectory to 300 yds sounds like something that you could get with jbullets, but
esp with a very heavy boolit, you are unlikely to get the kind of velocities that are
normally associated with 300 yd 'flat trajectories'.
Bigger diam, lighter boolit, start much slower and learn. You have embarked on
a master's degree level project as a newbie. I recommend you start at the
beginner difficulty level and work your way up. Like 16.0 Gr of 2400 at about
1600-1700 fps with a .310 or .311 150-160 gr boolit.
Bill
Bill