First, have you checked to see if your mount and rings are tight? That could cause a decrease in accuracy. After your rifle failed to fire the hornady ammo did you try another type to see if the same thing happened to that? Last what kind of a scope are you using?
I have a 223 ultra that gave me nothing but problems until i found an ammo type that it really liked. It shoots WWB 45gr hp into a 3/4" group at 100yds consistently. I tried hornady 40gr and 55gr, remington 55gr fmj and 55gr corelokts. Then i had trouble with a scope that had a wandering zero. It was replaced with a nikon buckmaster from a rifle that i sold so now it is my go to varmint rifle. I love it.