Mil-Spec buffer tubes — those made to military specifications — are forged to form a single piece with rolled threads. They are commonly manufactured from 7075 alloys and measure between 1.145 to 1.150 inches in diameter, making for a thinner yet stronger design.
On the other hand, you can also find commercial tubes made from lower-grade aluminum in 6061 alloys for a more economical option. These tend to be slightly bigger than their counterparts, with cut threads instead of rolled and a slant rather than a flat at the end furthest from the receiver.
When deciding between Mil-Spec vs. commercial buffer tubes, remember that little scientific information indicates one is better than the other. The most important thing to note is that a buttstock will only fit one or the other, so make sure your buffer tube and chosen stock are compatible before purchasing either.