Incidentally i have a buttload 7.62×51mm NATO rounds. SAAMI says these are interchangeable in .308 rifles. Any input?
As for ballistics, both internal and external, they are the same round (when comparing like bullet weights). The 7.63X51 was developed as the replacement for the old 30-06 round which was the US standard for rifles. As it was under development a US manufacturer saw the opportunity for the .308 to be marketed in the civilian market. It was introduced to the market about 2 years before NATO accepted the same round as a standard and called it the 7,62X51.
Differences like in the brass case itself. The case walls are thicker near the base to allow for more durability when used in machine guns or full auto rifles. Shooting 7.62 NATO rounds in civilian rifles is not an issue. The problems arise when reloading due to the heavier walls of the case which reduce the volume and can drive pressures above safe limits.
There is a lot of discussion about NATO pressure versus SAAMI pressure for this round. The difference in published pressures is due to the different methods for measuring the pressure, primarily the location of the pressure measuring device in the test firearm. Again, they are the same round, assuming the same bullet weight. The military standard is 147 gr while most civilian rounds are typically 155gr, 168 gr, and 175 gr.
As for performance, the .308 round is considered by most to be one of the best rounds for accuracy. I have shot many 5-round groups that look like a single ragged hole. Inexpensive to reload, my match grade rounds cost me approximately $0.40 each. Barrels last a long time with many seeing 5,000-6,000 rounds and no real deterioration of accuracy. Compare that with some of the .22 based calibers where barrel life is less than 2,000 rounds.
In short, a .308 rifle is very versatile. Inexpensive to shoot, easy to reload for, and accurate.