Seems your rifle likes the lighter bullets, given that you got sub-minute results from the 60 grain bullets.
Just because a rifle has a fast twist, and is capable of handling heavier bullets, doesn't mean it's going to "like" them...
My son's Rem 700 bolt in .223 HATES heavier bullets...
From what's on the net, you'd expect every AR to have half-minute accuracy. Bull...Expecting most "off the rack" AR's to shoot much better than minute of angle is unrealistic, as is expecting the same accuracy as you get from your 700 bolt gun. On the internet, if someone has a rifle that shot a 1/2" 3-shot group once...upon a time...when the moon was right...they immediately declare it a "1/2 moa rifle". Makes them feel good, I guess...but I know better.
Sounds to me, like you might be expecting a bit too much from the rifle.
It definitely makes sense to experiment with a different powder or two..
We tried H335, and it did not shoot as well as Varget.