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Just keep in mind that old sources were written when brass thickness was often greater (somewhere I have an old Peters 30-06 case that weighs 215 grains) in some cartridges, and primers and powder both were either differently formulated or were manufactured by different processes. Where manual maximum load data today is pressure-tested in SAAMI pressure and velocity barrels with chamber and bore dimensions held to within half a thousandth of an inch, the data in old manuals was developed by firing in productions guns with unreported tolerances and judged by reading the pressure sign tea leaves. In other words, cross-check against modern data wherever possible just to make sure what was published then isn't out in space somewhere as compared to modern data. The powder distributor sights generally have a good deal to compare it to that is free.
 

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Interpolation takes values off a trendline inside the range of known good data points, while extrapolation takes values off a trendline outside the range of the known good data points. Extrapolation is more prone to error than interpolation because you can't know if there is an abrupt change in the trendline outside the measured data range, as when a powder is spikey on the high side of the known good data range or prone to squibbing out on the low side. A trendline taken from good data doesn't tell you that. The point is, In both cases, you depend on some known-good data points to determine a trendline from which you estimate what other good data points are likely to be. The problem with old data is you can't count on its good data points to have remained good over time, so you can't count on constructing a valid trendline from it for either interpolation or extrapolation.

This is not to say all old data has gone bad. The same 45 Auto loads of Bullseye that were good in the 1920s work safely with hardball today. I can think of a few others, but the point is not to blindly trust any old data, as some products have seen more significant changes.
 
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