Nomercy,

You are absolutely correct in your formula for Kinetic energy, (K=1/2 MASS x VELOCITY squared) We substitute weight for velocity.

However, I think of this opposite from you. By reading the formula, one can see that the weight of the bullet is less important to the energy because to begin with, you immediately cut the weight in half. (1/2 MASS)
But, for velocity, you multiply the speed by itself, (velocity squared). If you plug in the weight and speed of a 30-30 bullet (Say 150 gr.) in this formula, and do the same for a .243 bullet of say 100 grains, the outcome in energy is very different. Try it, I just did along with my 8th grade pre-algebra class. The students, (especially the boys who hunt) found it amazing that the .243 bullet had more energy.

You could substitute the 170 grain bullet for the 30-30, but then the velocity is even less which hurts its over all energy even more.

I understand that as the heavier bullet travels, it retains its velocity better, but it doesn't start to make any difference until after 200 yards or so. I don't think many use the 30-30 much after 200 yards, but the .243 is still flying "flat."