Thank you, yes the M40A6 has the "catch all" 1-10 twist, I'm guessing the 20" vs 24" came about more for easier handling in urban environments and the faster twist was because of the velocity loss going from 24" to 20" with the M118LR ammo. No doubt in my Marine Corps oriented civilian mind that the armorers at Quantico did a lot of testing.
For me, it's a toss up,one range goes to 300 and another one goes to 600, 1000 if I want to build a portable bench. And, several of my old USMC shooting buddies have gone to help guard the streets of heaven, and I have inherited several shit pots full of 175's, pulled 118 LR's along with 168's and 155's.
Thank you
Semper Fi