I have a couple, both 14 twist. One is a Rock Creek, the other a Broughton, RC came to me because of my dabbling in product writing, the B because I misunderstood what a friend had told me when I went in with him and some others on an order. In both cases, my initial impression was not particularly favorable, but after my initial tries, and putting them aside, I took another look, and think that I have been able to do a better job of figuring out how to manage them. One clue came when I was running a patch into one of them. It seemed to enter the rifling with less effort, which made me wonder if that rifling shape reduced the force required to engrave a bullet. Thinking about that, and how 133 seems to like more starting pressure, one day I decided to abandon my preconceived ideas about appropriate charge weight, and venture upward until I found too much pressure or accuracy. At the time, this was done with some 62 grain Watsons, which were, looking back, probably not the best choice. Nevertheless, I found a node, at a few tenths over 30 grains (estimated by my measure setting) that while requiring all of the tricks needed to get it in the case, did produce excellent results. I believe that that was with the RC. Another experiment that I did that day involved securely taping a Possum Hollow trimmer flush with the muzzle which lowered the required charge to about 30 gr, which was easier to get in the case. When I removed the weigh, it returned to its previous measure setting. In any case, I had learned about one of the differences between that barrel and the others that I had experience with. Later, I applied that to the Broughton, and beyond that figured out that it wants to be cleaner than my other barrels need to be, to do their best work, so that I would have to remember to clean it more often.
The bottom line in all of this is that while I was able to get these barrels to perform on the same level as my others, that I found no advantage in them over those with more conventional rifling. If I were shopping for a slow twist 6mm barrel today, it would be a conventionally rifled four groove from one of the proven cut barrel manufacturers.
Going back a couple of posts, I concur with your .236 bore response - in my experience, there is nothing wrong with a tight bore, that a proper bore (in this example .237') won't cure. Customer feed-back has me gun shy of the .236 barrels - at least when I use J4 jackets - the bullet failure rate is MUCH higher, than via .237" bores from the same maker(s)!