Very familiar with the difference between SAAMI and CIP. Also pay attention to the difference between the various manuals. Let me state right here I believe you should buy every manual you can find. Case in point:
Hodgdon's and Hornady have a long relationship developing ammunition together. Hodgdon's Superformance and LeveRevolution powders were developed for Hornady. For the 375 Ruger the top loads of Hodgdon's powders differ between the two. Hornady is the only source of cases so that is not a variable. Which do you accept? I assume that neither is gospel in my rifle. I test every load.
I compared common loads with the same bullets (Sierra's) in both manuals and noticed that Lapua loads heavier charges, the real difference in components being cases and primers. Some of Sierra's loads use Fed #215 LRM and that change alone would require a reduction in powder weight as the Fed is one hot spark plug.
CIP measures at the case mouth so there is no factor applied for the cartridge case. SAAMI makes some (bold) assumptions about the pressure restraint of the brass case as the transducer is over the case and it absorbs some of the pressure. SAAMI adds a (large) assumed pressure factor to the raw data to arrive at the reported pressure #. I, like many others, think CIP is the more accurate of the two. And yes, their chamber/bore specs are different.
In my youngest son's 300WinMag, matching the Sierrra components, two loads, 150gr SpBT/ 75gr 4350 and 200gr SpBT/75gr 7828, both indicated (and later tested on a Pressure Trace II retrofitted to the
absolute max pressure. I have used Max loads from Sierra for 30/06, 45/70, and 556 that were mild, the 556 in a match chamber (NATO 556 throat). What's up with that? S.O.P. in my experience.
The upshot of this is an assumption I wrongly made thinking everyone thinks like me. I research all my loads from as many sources as I can find. The lowest listed maximum load is my assumed maximum until proved different IN MY RIFLE! The heaviest load I find searching out Load Data is my assumed "never exceed even if it tests safe" maximum.
This has worked for me for 40+ years of reloading. I still have all my fingers and both eyes. I raised three boys and, after their teenage years, my hair, hearing and sanity..... not so much....