From your post I understand that a 1:7 (or the fastest twist rate) will give you a wider variety of bullet weights that you can use over slower twists. But would a 1:12 twist be better for say a 55gr 5.56 than a 1:7 in terms of (all else being equal) groupings and accuracy?
I have read that someone makes a 1:6.5 twist but I don't know anything about them.
As too the 1:12 shooting 50-55 gr bullets better then 1:7, I'm not sure that is true, After shooting light bullets (Mainly 52-53 gr match bullets) I find my 1:7 shoots them as well if not better then my 1;12 223s.
The best bullet I've seen out of the Mann device was the 52 gr SMK, pushed by 25 gr. of 3031 and Remington brass.
If the 1:6.5 twist exist, I don't see the advantage, I shoot 90s out of my 1:7 quite well. The problem with the 80s and 90s is they don't fit in the magazine as they have to be loaded longer. Which is ok in match shooting because you single load anyway.
Understand its not really the weight of the bullet, but the length. Velocity helps spend the bullet faster, that we know. But in long range shooting, 600-1000 yards or more, you dont get the velocity you need, that is where the faster twist comes in.
You could probably get by with a 1:9 out of a 22-250 or 220 swift because of the velocity, but you can't get that kind of velocity in a 16-20 inch AR.