Newberry's system applies to all brass, individually, but not to mixed brass for a given load. Especially not in .308 and .300 WM where the different case brands have a lot of capacity difference.
You'll see his preference on his recipe page. It is all Winchester brass. No other. Winchester .308 brass (avg. wt. 156 grains for the last 1000 I got) has about 1.9 grains more water capacity than Lapua (avg. wt. 172.5 grains for my lot), and Lapua has about 0.8 grains more water capacity than Lake City (avg. wt. 179.5 grains; I have examples from 177 to 182 grains, depending on year and whether it is match or NATO).
That's just too much change to expect a tuned load to carry over. Lapua will want about 1.2% more powder than Lake City, while Winchester will want about 2.7% more powder than Lapua to get a matching barrel time. These are figured for Varget under a 175 grain SMK, but even though these ratios are rough for other powder and bullet combinations, they will be close enough across the board to save you having to fire as many rounds in new round robins when you change brass, knowing roughly where you will land.
Keep in mind that case capacity is not the only factor. Neck tension differences from different neck hardness will have an effect, too. Other things that will make you refire are: different lengths and jacket thicknesses on different brands of same-weight bullets, different primer brands, different COL's. Just as you have to knock loads down and work back up with any component change, the same will happen to Newberry loads.
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Last edited by Unclenick; December 17, 2010 at 08:38 PM.