According to the Manson Reamers Head Space Gauge Interchangeability Chart I got with my go gauges the 300 WSM is also go for:
270 WSM, 325 WSM, 338 WSM ...it says also: DO NOT USE for 7mm WSM. The chart also shows a gauge for the 300 RSAUM
( 7mm RSAUM). Not sure if this helps.
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