If you're not in a hury, I'd remove the gas block/FSB and then use whatever you have on hand to "get close". Use a caliper, use a drill bit, whatever. From that dimension, then I'd measure and order gauges covering .002/4" under-to-0.002/4" over in .002" increments. That way you don't have a bunch of needless gauges on hand.
I'm no gunsmith so I'm not too keen on ordering a bunch of gauges just to have them on hand to cover the gamut of whatever might come my way. Alternatively (for carbines, not sure what I'd order for mid-lengths, so the initial measuring would be necessary) I'd just order .060" - .074" or thereabouts in .002" steps and that should cover what I'd be interested in learning. Anything gauging above my max pin gauge would let me know I'm way overgassed and I ought to be looking at heavier buffers and stiffer springs.
Again, I'm no gunsmith, but I don't know how important it is to know your exact gas port dimension, but it would seem to be important to know if you are in the range of correct port size vs over-sized. Not really sure where that demarcation would fall, but I'm pretty certain that if I knew I were .010" over spec, then I'm in the overgassed range and need to take some mitigating action. Maybe .005" over lands you in this range, maybe .007" is the number, that's another discussion. If enough people reported their:
- buffer spring type (stock vs CS extra power)
- and how well the gun runs with all that considered together
then we'd develop a database that in time would lead us in the right direction. Maybe that's too much to ask.
This would probably be more useful for the non-Colt/BCM owners out there because you can probably assume that the Colts/BCMs are running in-spec, or very close to in-spec, ports. It would also validate whether or not the BM/RRA/Armalite/et al owners do indeed have over-gassed weapons or if these barrels too are in-spec.