Here's how Homer Powley, often called the Father of American Ballistics, explained it to me in 1963. You can't get the same BC from a .224 diameter bullet as you can with a .308 190 grain bullet or a .264 142 grain bullet because the twist would be too tight. When you start going above .308 caliber with very long bullets you get issues of accuracy in the leaping blasting torquing rifle you have to shoot prone without sandbags or tripods.
Homer showed that the optimum caliber for high BC, the purpose of which was to buck the wind, was about 6.5mm. He constructed, with the help of Bob Hutton, then the technical editor of Guns and Ammo, a match rifle using the .300 Weatherby necked down to 6.5mm for 139 grain Lapua bullets. He gave it to a guy named Al Gutta to shoot in the 1000-yard national championships at Camp Perry. Hutton later wrote, in the Gun Digest, that the round was so hot that one of the bullets disintegrated in mid-air. What actually happened was that Gutta, who was not really a championship shooter, misfired on the wrong target, costing him any hope of placing high. I was there and saw it.
But Powley and Hutton were basically right, although today we know that there is no point in having too much velocity. Most 1000 yard matches today are won by a 6.5-.284.
