Re: H4350 vs h4350 EXT... Different data /powder?
There are a lot of things that effect pressure besides the amount of powder:
The case volume (as derived from the case wall thickness... military cases often have less volume than commercial cases and the wall thickness may vary between commercial cases as well)
The friction of the bullet against the bore of the barrel (pure cooper bullets often require different powder levels for the same exact same bullet weight, for example)
The power of the primer (and primers vary from manufacturer to manufacturer, even within the same nominal size).
The free bore of the rifle's chamber
How close you seat the bullets to the start of the rifling when chambered
Any loads where the stated pressure levels are measured in CUP may differ significantly from loads whose pressure levels is stated in PSI. The earlier CUP system only record the effect of all of the pressure over time whereas the PSI system allow the tester to see the pressure as a curve over time. The later system gives the tester a much better insight as to what is really happening inside the case during firing and its test results probably are a better indication of reality. Many recent reloading manuals have had the loads adjusted after re-testing with the later equipment.
But in general I have found in the past that Hodgdon data typically uses larger powder loads than Sierra (???). It may be the difference in the testing equipment used to get the pressure data, both the transducers and the pressure barrels (??).
I start close to the staring load and work the load up while checking for accuracy. I often stop long before the listed max load and NEVER exceed the listed max load in the manual that has the hottest load. I have a dozen or so old and new manuals and research the data before I start reloading. But I defer to the latest manuals where there is a difference.