I'm sorry Muley. I had no idea you can't comprehend what I am trying to say. Of course barrel length doesn't change twist rate. You asked a question about how many twist a bullet would make at 25 and 100 yards. I responded with the two different twist rates you mentioned. (1:20 and 1:70)
And as it was pointed out by you and muzzlestuffer I have come to believe the faster twist is necessary because of the velocity of the lighter charge. (less velocity requires more twist).
So don't get our panties in a knot just because I am answering a question that you asked me.