The open-base is why Barnsness' article showed Nosler partitions shot well in a large groove diameter #4 rifle as well as the #1 he used in the test. He noted the groove diameter was .313" - which is smaller than the CIP standard shown above in kuduae's post.

I have a box of 156gr. blue-nose 6.5's that are a mere .263" in diameter, which falls right in line with that post noting many Euro bullets were undersized, long ago.