That is the same size as a milspec buffer tube.
I bought this based on Ethan C. answer that it was a mil-spec tobe and it is not. I made a mil-spec fit by filing the flat portion down. The stock I wanted to use is only available in mil-spec so I made it work. I filled down the flat portion of the tube a little at a time. It is still snug but works fine.
I bought based on Etan C. Answer of it being Mil-spec but that is wrong. This is a commercial tube. You can get certain Milspec stock on but it is very tight. I ended up just keeping what I have despite the very tight fit cause it will work just not as easy if it would have been the correct match with the Mil-spec stock I bought.
It’s comm spec. Period.