Re: 6mm Twist rate
You can always shoot shorter bullets than the twist was designed for. It might not work as well as if it were a slower twist but will work.
In extreme cases of tighter than needed twists, pressure can be increased. Watch your loads. Typically, the preasure increases, and the velocity decreases. That the biggest problem you will have.
Long range shooting is a process that ends with a result. Once you start to focus on the result (where the shot goes, how big the group is, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.