Measuring the barrel gas port size
While I am generally opposed to the amount of attention paid to this level of minutiae, it strikes me that if we're going to discuss it so often we should determine how people are making their measurements and assumptions.
As I understand it the proper method is to use a set like these.
Further, I understand the "plus" and "minus" to mean that they are just slightly over, or under, sized relative to the stated diameter of the pin. 0.0002" is the over/under I've seen reported. So a 0.060" minus pin would just barely fit in a true 0.060" hole while a plus would not.
Additionally, I think that if you are the one reporting a hole size that you measured you should be stating how you took the measurement, and if you are repeating a size you saw reported somewhere else you should be linking to that other place or at the very least stating your source.