I've read Dan Lilja's comments about three groove barrels. If I remember right he had talked with varmint hunters looking for more barrel life, and the thinking was if the number of grooves was reduced to three while keeping the same land to groove ratio the barrel life could be extended because the same amount of steel would have fewer exposed edges to be eroded during firing. Cleaning could be easier with fewer corners to clean out.
From other conversations, the main knock on three groove barrels I recall was they were tougher on long target bullets in tight twist, high velocity situations. After the barrel wore just a bit the bullets would get torn up an come apart in flight, more so than with six groove barrels for example.
I don't know how the land to groove ratio varies between different groove numbers for Pac-Nor.
