Different powder ranges between manufactures.

[FONT=&quot]Cal .243.[/FONT][FONT=&quot][FONT=&quot][/FONT]
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[FONT=&quot][FONT=&quot]Lyman 49th ed suggests 85gn HPBT, COL 2.615”, Vhit N150 35 - 38.2gn[/FONT][/FONT]
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[FONT=&quot][FONT=&quot]Vhit own app data suggests 85gn Nosler Partition, COL 2.677”, Vhit N150 29.3 - 35.2gn. [/FONT][/FONT]
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[FONT=&quot][FONT=&quot]How can the powder load be at such odds between two reputable sources for a bullet of the same weight?[/FONT][/FONT]
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[FONT=&quot][FONT=&quot]The starting load for Lyman is the max load from Vhit.[/FONT][/FONT]

Two bullets for a start. Even though the same weight, they may generate very different pressures (bearing surface diameter; jacket thickness; core alloy hardness; and most of all bearing surface length).*

Then there is the case. Viht uses Lapua whenever available, while Lyman normally uses thinner and higher capacity Remington brass - that easily makes 1gn or so difference in the maximum load for the same pressure.*

Then there is the primer. Viht doesn't state the make these days, Lyman tends to use the Rem 9 1/2 - the hottest model on the market (half to full grain difference possible here)*

Finally, the pressure barrel used for the tests - those used in pressure measurement are standard barrels from one of the main barrelmakers but as close as possible to SAAMI or CIP spec in the bore / groove diameters. Like those in different production guns though they are all individuals and may generate different pressures and velocities. Although both organisations use modern strain gauge or Piezo crystal measuring systems hooked up to a computer, CIP and SAAMI use quite different methods and technologies. Lyman being American will use the SAAMI approved method; Viht as European will use CIP - that alone could easily account for a couple of thousand psi differences in readings for the same load.

* That's why loading manuals nearly always have a disclaimer that the load applies to that combination of components and may not apply if anything is changed.

Thanks Laurie, that explains everything. The two guys who have been giving me advice were telling me that the powder type and charge were all based on the caliber and bullet weight.