Receivers made to Milspec dimensions (almost all are this way) will take both the commercial tube and the milspec tubes. The thread size is the same in both tubes.
The thickness (diameter) of the commercial tube is 1.17 inches, and the milspec is 1.1465. There can be manufacturing differences between the two as the milspec tube has been manufactured to a standard. Milspec has forged threads, cold formed or something like that. The commercial tubes are usually cut threads. Some say the milspec tubes generally are stronger due to their construction specifications. That's beyond my expertise level though.
I think the confusion comes with assembled lowers or complete rifles. Some brands like Colt, LMT, Stag and others come with milspec sized tubes. Other brands like RRA and others come with commercial tubes installed. So if you upgrade or change carbine stocks you need to know what your brand gives you, or measure to be sure.
If you're starting with a stripped lower, you can put either milspec or commercial sized tubes and stocks on the lower.
