This is the method by which CALIBERS are listed:
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Your suggestion of .308" is simply the diameter of one .30 caliber bullet.
.300 SAUM is the name of cartridge per SAAMI. Technically it's still RSAUM for Remington Short Action Ultra Mag. Reading and learning is fundamental. Trying to rewrite the definitions of terms just to suit your desires is frivolous.
The greatest misuse is the term
'caliber' when the writer really should be typing
'cartridge' which specifies the loaded cartridge case by name.