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Is it possible that the difference in OAL is responsible for what appears to be a discrepancy?
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No way to know for absolutely sure, but probably NO!.
Possible differences include:
- different powder lots
- different primers
- different bullet length
- different testing equipment (how the pressure is measured)
- different firearm
- different atmospheric conditions
and so on and so on. It isn't a discrepancy, it is just a fact of load development and pressure generation.
The 9mm is sensitive to minor changes in seating depth (which is what really changes pressure, not OAL). But all the above reasons are why you always start at the beginning load and develop it for your firearm and components. And also why you should always check multiple sources before selecting your beginning point
Excellent question!