I find that there is a difference between the stated twist from the factory for a caliber and the actual twist that was reamed into the barrel on any particular day.
It can vary by maybe a 0.5 quicker than spec which is enough to allow a 77-grain high BC bullet to stabilize.
I have two .223s that shoot 77 SMKs and TMKs great; one shoots them better than any other caliber.
You really have to test performance in your particular barrel.
I have long since stopped assuming that each barrel will perform like someone else's.

Experience can be misleading, too.
The same rifle that shoots 77 better than any other caliber doesn't shoot some 75 gr bullets very well.
I think it may depend on the bullet shape as well as the bullet weight when you are near the limit of the twist.