You will have to remove the ejector to check this.
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Yes depending on what gauge you may have to remove the ejector then you place the go gauge in the chamber and close the bolt eensure that the bolt fully closes then try the same thing with the nogo guage and it should not chamber if it does then you have excessive head space and will probably need a new bolt. But like I said most often you will not need a new bolt. As for Bushmaster not being on the same lower than pond scum level as Oly or hess that is most correct and like I said I do it for peace of mind and will do it no matter who the manufacturer is. Because you never know who was QA that day it might have been Homer Simpson or someone having a bad day.
Assuming it was a quality bolt, I would expect it to be fine headspace wise. But being that he's using bushmaster components....who knows, even after just 250 rnds!;)
Have no idea as to frequency of headspacing issues involving bushmaster guns and or bolts. I have used a Daniel Defense bcg with two different barrels with no issues and did not use a gauge either. Why use a gauge? Either the bolt will lock or it won't....correct? and as you stated, there is no user remedy other than getting a new bolt or barrel extension.
if the parts are made to spec then headspace is rarely an issue, but how do you know the parts are made to spec unless you check?
You can use an unfired round as a rough check