That 1-18 twist barrel was a very limited run and expensive if I remember right. I don't remember the actual count but it was very small. If I remember right the story was a group of long range shooters got GM to put these barrels out. They are actually a 1-18 twist .458 so they are a BPCR barrel that they made to work in a ML. Anyway I got wind of this unicorn and ordered one. I shot it quite a bit but that 1-18 twist wanted a longer heavier bullet. I picked up some 550 gr .442 paper patch bullets when I was at Buffalo arms last month. I think they have a lot of potential. I also have a couple platinum lined nipples to try with them.
I think there could be more unicorns in different flavors but I don't know about them. My stainless 45 is actually a .451 the .458 is a 32" barrel but like I said I would not base length and twist together when one is a unicorn.