Oh, Wise Ones, A little help please.

[Terrill]

I have two problems and could use some help.

First, I was at my range yesterday trying out two different .300 Fireball AR uppers with mixed results.
One was a review rifle and it would break into the golden MOA barrier every now and then but only with loads well below max.
The other belonged to a friend and in every five shot string it would throw one round 2 to 3 inches away from the group. It might be the first shot one time and the third or fourth the next time. There wasn't any pattern. All loads were from the Sierra manual using 125 grain Sierra bullets with either H110 or 296. Any ideas?

The second item I could use some help with is putting together a combination for a new barrel. I want to put together an AR in .300 Fireball to obtain the best results using only 125 to 150 grain bullets at supersonic speed. Most barrels I have seen are in 1-8 twist, would I be better off with a 1-10 or 1-12 for this upper? Would a carbine gas setup work better than the pistol? This will be a single purpose rifle for accuracy and speed.