What are the required twist rates for barrels of different legnths to shoot accurately some of the new heavier loads like 69,75,77, and 100gn stuff?

69gr requires a 1:9
75gr & 77gr can shoot OK out of a 1:9 (depends on the barrel) but they work better in 1:8 or 1:7.
80gr?  definately 1:8 or 1:7

After having read most of the AR15 accuracy books (by: zediker, martin, feamster etc.) I've found in my tiny little mind that not only is the twist important, but so is the chamber dimensions. That's why a Bushy varminter should shoot better than a regular bushy with some bullets. They have different chambers. The free bore before the bullet hits the lands and grooves counts for much.

Another wild thing about bullets: It's not the weight after all, but the shape, the ogive. So some bullets of the same weight but with different ogives would shoot better with different twists.


My brief and twisted understanding of a somewhat complex subject.

M4-AK

Not only is the above important but so is barrel lenght since it directly related to velocity. All things being equal a slower rounds will need a faster twist to remain stable.