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That's .009" of crush-up the M240 provides with a maximum case/minimum chamber.

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NO-GO does NOT measure safety.  NO-GO measures the maximum chamber size that a NEW chamber should be.  If a NEW bolt and barrel close on a NO-GO gauge - something is out of spec.  However, some 5.56 NATO chambered barrels will still close on a SAAMI NO-GO gauge.... they are simply being chambered longer than they really need to be.

A FIELD gauge measures the maximum chamber dimension considered to be serviceable.  It is not necessarily a safety issue, it simply means the chamber size has worn beyond what is considered serviceable and should be replaced.  

When barrels and bolts are NEW, the chamber dimension should be somewhere between a GO and a NO-GO gauge to be considered within specification.  When a chamber can lock on a FIELD, it simply means it is worn beyond its service life, and should be replaced.

This is not a "safety" issue really, as the M249 shoots the same ammunition, and will SWALLOW a FIELD gauge when brand new, by design.  When your chamber is longer than a FIELD gauge, you do increase the chance of a case head separation, however, because the long chamber allows for the brass to stretch much, much more.  The problem with a barrel that will close on a FIELD gauge, is you have no idea how long the chamber is.

The reality is, most people should not bother with headspace gauges on an AR-15.  It is largely a waste of time, as there is nothing you can really do about it as the headspace is set by the barrel manufacturer.  If swapping bolts solves your problem, you never really had one, as you were right on the edge of the GO or NO-GO measurement.  Either of which are fine.  Buy reputable parts from reputable manufacturers, and go shooting.

failure to check headspace could result in injury or death

I'd not be a bit surprised if it says somewhere that according to Proposition 65, this product has been known to cause cancer in the state of CA.

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There is no skillset, knowledge or test required to call yourself a "armorer" or "gunsmith". In many states, it's harder to be a barber or cable tv installer than it is to be a gunsmith. The same applies when it comes to being an FFL holder. You can get a FFL 07 and never have even fired a rifle.
If I contracted a build from a FFL07, I would certainly expect them to check things like headspace. What someone does in their basement build is up to them. It's probably more important to know what the actual risks are from excessive headspace. I think the risk of injury or death in a 5.56 build are incredibly low. Damage to the bolt, firing pin and chamber, sure, but can the same be said about other calibers or chamberings, like a 458 SOCOM? At some point could you blow out the rear of a receiver extension if you fired a 458 SOCOM that had excessive headspace and broken, sheared or missing bolt lugs? It would be interesting to see if someone did any tests on intentionally excessive headspace trying to get a kaboom in various calibers.

Damage to the bolt, firing pin and chamber, sure, but can the same be said about other calibers or chamberings, like a 458 SOCOM? At some point could you blow out the rear of a receiver extension if you fired a 458 SOCOM that had excessive headspace and broken, sheared or missing bolt lugs?

It would be interesting to see if someone did any tests on intentionally excessive headspace trying to get a kaboom in various calibers.

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Keep in mind also that for weapons with barrels that are fixed at the extreme rear, the chamber headspace dimension will grow as the barrel heats up, so just because everything is running like a sewing machine when you are at the range popping off 1 round a minute doesn't mean that it wont crap out on you during mag dumps.

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Injury and Dealth?  And I suppose you don't think a lawyer had a hand in writing that statement?  I'd wager lawyers had just as much input in the writing of the Manual as did actual armorers because they knew too many people in the civilian population would take that manual as gospel from God so they had to include every CYA statement they could think of.  I'd not be a bit surprised if it says somewhere that according to Proposition 65, this product has been known to cause cancer in the state of CA.

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I'm not sure.  I would prefer that it didn't close on a no go gage.

Having the gun not work because of a case separation due to excessive headspace is certainly a safety issue, if you are using it for what it was intended for.

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Having the gun sieze up due to a case separation is certainly a safety issue if you are in a fight, and would likely be fatal.  I don't why you are treating that like it's no big deal.

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 If you have excessive headspace so extreme that it could cause a problem, your fired cases will indicate it.

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So firing off rounds w/o using gauges to see if the cases indicate an issue, is a better option than using gauges before firing a round? I understand the technical aspects of both, but your logic is not adding up for me.

Why is gauging the bolt-barrel set such a voodoo thing to do when it's a DIY contraption of sorts?

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WTF did you get that idea.  I never said it was a bad idea to check headspace.  It can be done without gauges with a FL sized case and some tape, which is my way.  My point in all this is that excessive headspace is not going to blow up your frigging gun.  That is all.  Jeez