6mm ARC vs 6.5 Grendel Sanity Check

[LRRPF52]

i recently built two 6arc uppers. one 16" from sixfive arms and one 12.5" faxon gunner barrel.
my velocities are as follows with hornady 105 bthp black. 16" 2515 fps, 12.5" 2402 fps. the elevation is roughly 3200'

That Hornady data is probably based on the 110gr limits.

The .5” extra barrel length and 105gr makes sense for the velocity that you’re seeing.