Difference between "Commercial" and "MilSpec" parts??

[J]

Its measuring OD.

The biggest difference is that in a commercial tube, the tube starts wider, and the main body is untouched, and has the thread grooves cut in to it. So the tube is the same diameter as the 'teeth' or 'high-points' of the threads. On the milspec tube, the entire tube is cut into the blank, so the main body has the diameter of the 'grooves' or 'low-points' in the threads.

In other words, look at the threads...
The commercial spec has the threads cut down and recessed in to the tube, the milspec has the threads protruding from the tube.

Comm-spec can be made cheaper, and lighter.

The mil-spec get slightly deeper threads and is slightly stronger.

That said, comm-spec tubes are still really dang strong if made well.