I recently got a 15 twist 6mm barrel from a fairly prominate barrel maker. It was stamped 13.5 twist which is what I ordered. The barrel teasted me until I took it to Raton and it shot like a lazer at 6000+ ft. Much better than I usually am able to shoot. But back at 1100 foot altitude it would keyhole bullets. I finally had the light go on and measured the twist. Duh! It was right on the edge. The company replaced the barrel, but not my chambering costs or shipping costs to return the barrel to them. What is it that they say. Sh!t happens. No one is perfect. I don't hold it against them and have since ordered an aditional barrel from them. But from now on, I will check the twist before doing any chambering and would suggest that others do the same.
Donald