I used to have a custom AR-15 with a Shilen 1:12 that would stabilize the old 60 grain Nosler solid base. I think it was about at its upper limit, though. They say the 62 grain NATO SS-109 is marginally stable out of a 1:10 - solidly so with a 1:9.

Called Sierra Ballistics this afternoon. They said "Hogwash" at the notion that a bullet could be unstable with too fast of a twist. They did, however, indicated that a bullet with way more twist than it needs doesn't fly well at real long range. I guess it tends to continue through the air at the angle it left the bore - and when its dropping at a 15 degree angle at long range, its still flying through the air with its nose facing a little uphill - can make an oblong hole in the target, even if it is completely stable.