Yes, Richard is building 120 gr ULD RBBTs in 6mm, also 110 and 115 gr ULD RBBTs.
I have not tested the 6mm Wildcat Bullets yet but after the 224 Allen Mag is finished up(just got the reamer in [img]/ubbthreads/images/graemlins/grin.gif[/img]!!!) It will be necked up to 6mm and a 6mm Allen Mag will be released using these three bullet weights.
While I can not comment on these specific bullets I can say all the other extreme weight Wildcat bullets I have tested in several calibers have performed extremely well in accuracy at long range.
You would have to contact Richard at firstname.lastname@example.org
and ask him what twist to use.
On the 6mm-06 issue. If you are going to use these very heavy bullets you may even consider the 6mm-06 AI. I have this reamer and with really heavy bullets it performs very well. In fact the heavier the better. Would make a true +1200 yard rifle with these heavy 6mm bullets.
The standard 6-06 will top 3500 fps with a 107 gr bullet in a 30" barrel rather easily. I have built several and they are all great performers in sporter weight or heavy rifles.
Please keep in mind that no matter how fast you drive them you are still dealing with a 6mm bullet. Ballistically they can be extremely impressive but remember that energy is always a limiting issue as is spotting bullet impacts at extreme range with these smaller bullet calibers.
That understood, they can produce very impressive results at 1K.