I'm also interested in long range shooting and plan on purchasing a rifle soon. I'm also no expert on scopes but I have used the BDC type reticles for a while now. So with that being said, unlike first focal plane scopes the BDC reticle on a second plane scope is set up to be used at the scope's highest magnification. Now if you want to stretch that yardage to use, say the last (bottom) aiming point for 600 or 700 yards you can dial down the magnification until your bullet impacts where you want it and either mark the scope or make a note of the magnification position.
You may find that at a certain magnification point your BDC's aiming points may change from 100-500 yards to 300-700 yards. Just guessing. And if so that would be great for your 300RUM if the ballistics will allow you a 300 yard zero.
It would be interesting to hear your results.