I say it's necessary to disassemble the bolt to check headspace. The brownells field gauge is a Forster product. I will admit that it can be a pain if you don't have experience dealing with the ejector roll/spring pin.
All of my gauges are modified so bolt disassembly isn't required. When a bolt/barrel combo fails, I always follow up with other gauges to verify my gauge is not off. I also check the calibration on my gauges every 6 months.
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