As far as
@Potsie question.
The 25-30TC (25CM) is SAAMI speced with a faster twist rate than the old standard 25 caliber twist rate of 1:10. Meaning you can stabilize heavier bullets.
Otherwise it would top out on the 120gr bullets that the 1:10 does.
So people that don't (or aren't into) custom built rifles can get one over the counter.
Buy a prefit barrel with a 1:7.5 twist and screw it onto a Savage action in 250AI, and you would have a ballistic twin.
Put a 1:7.5 twist barrel on a 257 Roberts AI or a 25-06, and it's an entirely different beast!
As for why 25 caliber over 6, or 6.5mm?
For some unknown reason the 25 caliber seems to kill above what it's caliber would suggest.