What's the difference between the 6mm Grendel and the 6mm ARC?

[Harpoon1]

For a more numeric comparison try post #32 here http://www.65grendel.com/forum/showthrea...ht=6mm+ARC

When did the Grendel get a “60 degree shoulder”? Think you have a math error brother! Same thing for the 6 PPC! Both have 30 degree shoulders, just like the 6mm ARC.