Loads for 10" AR15
 

I'm having trouble with H110.

Here at 4000 feet elevation, the speed of sound is about 1150 fps on a summer day. I'm trying to work up a subsonic load for my 10" AR15.

10 grains of H110 pushing a sierra 220 will do 1115 with 6fps standard deviation. However, at 50 yards, that load will only group 2+ inches. I can't figure out why. My brass is all the same trimmed length.

I'm using winchester small rifle primers.

What should I do for better accuracy? I'm thinking magnum pistol primers might be a good start. Is H110 just a poor powder for my purposes?

Papa

I'm not sure what you'll get out of a 10" bbl, but try 8.0-8.5gr of Alliant 2400. It is in the same burn rate class as H110, but is a flake powder, which will probably give you consistent ignition with standard primers.