Originally Posted by Varminator 911
I'm thinking now the small calibers are limited mainly by the fast twist needed to launch long bullets and the larger calibers, say 375, are limited mainly by heavy bullet weight.
Yes. The advantage larger calibers have is that higher SD bullets can be made less "long and skinny" than smaller calibers. To match the SD of a 338 cal 300 SMK, for instance, a .224 would need to be 132 grains. Using similar construction, this would basically be as long as a pencil and require a silly twist rate to stabilize, one that would likely rip the bullet apart while any old 1:10 works for the 300 SMK.
So if you stick a small bullet in a "Honey I Blew Up the Bullet" machine to take it up to a larger caliber while keeping everything else proportional, you end up with a bullet that has a higher BC. This is because the SD goes up--the cross sectional area goes up with the square of the increase in size, weight goes up with the cube. And being the same shape, the CD or Form factor will remain basically equal.