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If you had the same gun as used for testing using the same cases, primers, bullet lot number and powder lot number, you might get the same results as the manual.
If you are using a different bullet, it will have different geometry. Powders do very lot to lot.
For safety, and I know I'll be laughed at, I use the LOWEST starting load I can find. I may not fire more than a couple of rounds, but I prefer that to being surprised.
This effect has been reduced over the last decade as all testing by SAMMI is done with transducers that plot pressure vs time and spot any pressure peaks or other problems. So, today, a reloading company might spot a pressure peak outside SAAMI limits in their tests and thus lower their max load.
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