With headspace gauges you can always use the same gauge for an entire family of cartridges a headspace gauge measures the shoulder headspace (at least for rifles it works that way). You just need to make sure that the shoulder of what you are measuring is at the same place. For example, a 270 and a 280 are both based off a 30-06 but the shoulder of the 280 is different so we use the same gauges for the 270 and the 30-06 but we need a different set of gauges for the 280.
The 221 and 300 use the same shoulder so we can use the 221 FB to check a 300/221. Here are some 221 FB gauges:
http://www.midwayusa.com/eproductpag...eitemid=223971
http://www.midwayusa.com/eproductpag...eitemid=255539
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