From what I recall, the inner and outer dimensions of the buffer tube are the same. The real difference is in the way the threads are cut on the mil-spec tube as opposed to the commercial version. I have heard of guys getting a mil-spec tube in a commercial receiver and then can't get it back...
My daughter asked me about how unions work. I told her to imagine that I am a potential employer and I want to hire you. This is a very good job but you have to pay your sister $100 every month or I have to fire you. I did not pick the $100 figure arbitrarily. That is what I pay each month. I...