Tell us about your Midwayusa AR Stoner barrel.

[LRRPF52]

Have you measured with factory SST? Hornady says 2350fps MV from a 16in barrel.

That AR15 provided to Hornady was a 14.5" with permanently attached muzzle device bringing it to 16" length. If you look in the Hornady 8th Edition, it shows that they used an AA 14.5" Grendel. Guess what the max velocities are for the 123gr A-MAX in the 8th Edition with A2520? 2350fps

My 16" Grendel shoots Hornady 123gr Factory A-MAX at 2460fps, but you really could have enough variation from barrel to barrel to account for these differences. Hornady's site still lists 16" and 24" velocities for their factory loads, with the 16" being at 2350fps.