And I didn't say that belted magnums were not accurate. If you re-read my post, I said that consistent belts, or neck sizing can compensate the the potential headspacing problems.
But since not all shooters handload, a valid comparison must use factory loads. In that case, any inconsistency in the length of the belt will effect headspacing, bullet travel before engaging rifling and therefore accuracy.
My hunting handloads for the .300 WinMag (165 Speer BTSP) and .338 WinMag (250 Speer) are very accurate. Sub MOA. But I mic the belts, and turn them to uniform within .001.
If I ever get another magnum (doubtful, since I now use the .30-06 for everything
) it will be .300 RUM, so that belt turning will not be necessary.