I’m a 30-06/.308 nut. Dunno if any of the following is useful to you or not, but here goes. …
A .308 Winchester is basically a shortened 30-06.
In metric, They could be called 7.62x51mm and 7.62x63mm respectively.
Additionally, the .308 has the same outer dimensions and neck thickness as the 7.62x51mm NATO. Case thickness is different in the head and part of the wall, so the NATO version has less case capacity. Pressure ratings are also different: the NATO version having less pressure than the upper limits of those allowable in the commercial .308 Win. From the outside, they are the same and are (for the most part) interchangeable. Usually the difference is not a problem unless you’re reloading or wanting to use heavier bullets in military semi-autos(that weren’t designed to handle the higher pressures of some commercial loads).
Some commercial loadings in 30-06 more pressure (or the wrong pressure curve) than the military loadings of 30-06.
Basically, you can’t feed a Garand a diet of heavy bullets without tearing it up and the same goes for military semi autos "chambered in .308". There are similar differences between the .223 Remmington and its military sister, the 5.56x45mm NATO. What matters here is not only the peak pressure, but the pressure at the time the bullet reaches the gas ports.
The .30 TC is another step in decreasing the case size in the family of cartridges based upon the 30-06 case.
What’s happened over the years is that improvements in propellant technology have allowed the case to be smaller.
The .30 TCU is different, and based on the .223 family of cases. I had never heard of it or the .30 TC until this discussion.
To measure case capacities, I pour known volumes of sand (or powder) into the case. The "measure, mark, measure" method for deciding where to cut one.
Keep smiling ... it'll just make 'em wonder what you're up to...