not magic, the 200 gr partition is ever so slightly smaller diameter. they do this to reduce friction that would be greatly increased due to the unholy length of the bearing surface on that bullet. the 200 bullet going the same speed as the 180 sacrifices a tight fit in the .308 bore for that speed, some rifles will shoot them VERY well but the majority will not. not trying to discourage you from trying, just stating why. have tried them in .308 Norma (very simmilar to the WM) with no luck
Is .3075" under sized in your opinion. It is not in mine. It has shot well in every rifle i tried it in, but it is a hunting bullet. Never designed to be a match bullet. The 10 elk i have killed with them proved their performance.