A little thought will reveal that this issue is not about the Speer and Lee brands, but about geometry. If the ogive radius of the bullet is long enough and the bearing surface short enough, you can run out of adjustment in the Lee Dead Length seating die because its seating stem is stopped before reaching the case mouth. It doesn't matter what the bullet brand is, just what the geometry is. For the vast majority of common bullet shapes this won't be an issue.
(BTW, it was not Speer, but Sierra that had the advertising grudge match going with Lee long ago. It was the Lee Factory Crimp Die being applied to match bullets that Sierra objected to because it indents the bullet profile. It was not over seating dies. Lee said, try the FCD and see for yourself. Sierra objected some more, so Lee finally got annoyed and started to advertise the FCD worked with all bullets except Sierra bullets. Can't really blame either side. Both valid perspectives. Some percentage of the time the FCD does improve accuracy in some rifles with some loads, at least to moderate ranges, by increasing consistency of bullet pull. I expect the slight indentation to cause a small decrease in ballistic coefficient and, if not perfectly symmetrical, some increase in wobble, so I doubt 1000 yard shooters would want to mess with it. It just depends what you are doing and how you use it.)
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Last edited by Unclenick; February 12, 2013 at 08:02 AM.