you know one in nine inch twist in a bushmaster is really 1-8.75"
i shoot heavy hornady match 75 grain bullets out of the 1 -9" barrels from 11.5" to 20"
though i don't have all brands and makes of barrels to me it seems most 1-9" barrels can run up to 77gr length bullets and still be stable.
the shorter the barrel the more likely the bullet will become unstable.
even the rather short 11.5" so far has keep the stable enough to be accurate out to its max range of 200+ m -- "knock on wood"
the 69 gr bullets seem to be the favorite of the 1-9" shooters --some barrels like it better and group tighter with it than the 75-77s