I bought a Diamondback AR10 308 w/ 18" barrel. Had the same trouble....3-4" @ 100y regardless of what I fed it. I even broke down and bought a box of Federal Gold Metal Match 168's....supposedly, if a 308 won't shoot them then something is wrong. Well, something was wrong. I called Diamondback and explained everything I had done, they sent me a new barrel. I got it and put it on....now it'll shoot groups that you can cover with a quarter at 100y.
Point being...don't be afraid to call and talk to the manufacturer.
Interesting you say that, this is a diamondback, which i purchased earlier this year. I wish I had shot it sooner so I could see if the barrel is actually the issue. How long did the process take between sending the barrel in and receiving a new one?
They were great to deal with. I had the barrel with a prepaid to send the old one back to them (it was the stainless barrel, had it been the chromoly they acted like they wouldn't have wanted that back) in less than a week. I swapped it and started load work up again.
Also, it's not the weight of the bullet that the rifle likes or dislikes. It's the load used with that bullet that the rifle likes or dislikes., So, saying that your rifle won't shoot a certain weight bullet is false (generally). Selecting a tangent ogive bullet that is appropriate for a AR will also help.
I agree with this to a point. 1 - weight of bullet is not relevant, it's more type/style/brand/etc 2 - In my experience life it too short to force feed a barrel a bullet that you have to jump through hoops to get to shoot. You can also get to a point where you are throwing good money after bad. (been there done that...got a 1k 130 Berger Match 6.5 bullets that are useless to me to prove it
In short what I'm disagreeing about is not every rifle will shoot every bullet....sometimes it's not the load it's just that barrel isn't jiving with that bullet. Ex - you get a "fat" or "slim" bullet in a "tight" or "loose" bore and they just won't work....Lapua Scenar 105's in a .237 bore is typically not a good recipe for success.