I finally got this sorted. Fastenal had M14 and M16 but no M15 to try. I searched all over the Blaser website and all threads listed were metric. I contacted Blaser but never heard back. I called Euro optic ( the people I purchased it from) and it took them a few hours to find out it was M15-1. Which as it turned out, it was not. So I measured it with my calipers at 0.625 inches. Turns out it is 5/8-24. The threads have the same nitride finish as the rest of the barrel, so it must have been done at the factory. They must have known it was going to the US and put standard threads on it.