There may be something to this subsonic stability factor. I have also heard that when a bullet passes below the sound barrier at long range, it can have a temporary effect on its stability - sort of like the bullet had to break through a thin sheet of plexiglas. But if it was never above it to start with . . .

I still disagree with the RPM theory. I have, over the years, rechambered quite a few small case .22 centerfires to big case .22 centerfires. Often times, a benchrest shooter will have a premium barrel in .22 PPC or .22 Rem BR that has opened up its group to .3" - no longer competitive in benchrest, but still a fine varmint barrel. So the take-off barrel is rechambered to .22-250 or .220 Swift and put on a regular varmint rifle. It's still not going to stabilize a bullet much better than the smaller case .22 CF. A little better, but not much. If it is a 1:14 twist, either the .22 PPC or the .220 Swift can stablize the 55 HPBT - and neither will work with the 60 grain Berger. And that's going up in speed almost 1,000 fps.

If the RPM theory held any water, bullets would suddenly become unstable as they slowed down in flight to the target - the faster ones would just get further downrange before encountering this in-flight instability. And we know this doesn't happen . . . .