boltman223, please reread my longish explanation above. A 1:7 twist will statically stabilize anything up to 80 grains or so in standard length barrels, but they will spin the bullet faster than required and that may lead to some issues as I described above. There is no need to spin these bullets that fast, but given the choice between a 1:9 and a 1:7 and wanting to shoot the longer heavier bullets, you want a 1:7 not a 1:9.
Many of the ARs come with a 1:7 twist barrel, because that's "milspec" and yes, the bullets you mention will stabilize properly in those barrels. 1:8 twists, arguably the best twist for a .223, are fairly rare, found only in match guns usually. And these are certainly not "milspec," so nobody asks for those, the attitude being "if 1:7 is good enough for the military, it's good enough for me."
As for velocity, yes, you can certainly make up for a not quite fast enough twist by increasing the velocity and thus the spin rate at the muzzle, but that will only make up some twist and then the increase in the overturning force will require a faster spin and you just can't make it go fast enough. Our favorite batracian is able to make the 90gr bullets stabilize in a 1:7.2 twist because he has a long barrel. If the bullet is stable at the muzzle, he's good to go because as I explained, the bullet only gets more stable the further it goes.
In normal barrels, like 20 to 24 inches, you will not be able to stabilize the 90SMK with anything slower than a 1:6.5 twist, as stated on the Sierra website. Now, maybe the 90 Berger is shorter than the 90SMK and that would help stabilize it, but I have neither of these bullets and I am too lazy to look them up in the bullet database right now.
