Grain of bullets. When using different grains of bullets, what are the main differences is it speed v. power. For example if i usea shoot a .270 with 130 grain bullet, I am assuming the bullet will fly faster than say a 150 grain but won't have the same amount of power behind it. . However with the same gun ,the 150 grain will have more knockdown power but won't fly as fast. Am i right on this? What are the main differences between grains?