I have a civilian .223 FIELD headspace gage (got it from Brownells, I think its a Colt). I've never used it as its a hassle to remove the ejector on my AR15s prior to making the measurement. I hear that the military headspace gage has some kind of clearance cuts so that it can be used with the ejector still in place.

Is is possible to modify my civilian gage to work in the same way ? I am thinking I can just grind off some material at the base of the gage to provide clearance for the ejector plunger. Is this gonna work, or is it a foolhardy exercise ? Does anyone have a photo of a military gage I can use to guide the size/shape of the cut ?

The military one is just thinner where it meets up with the face of the bolt.  A machine shop may be able to turn yours down.  Mine is .22" at the base of the gauge.  Which allows you to leave on the extractor and ejector.

Thanks for the feedback. Could anyone post pics ? I hope I can do this with hand tools (i.e. just grind off the offending metal).