Re: 1:9 twist max bullet weight 223?
It's not just barrel twist which determines the stability of a particular bullet Air density and velocity play a part too. Higher altitude, higher temperature, and even higher humidity all give the air lower density and help a bullet to stabilize. The higher drag of a faster bullet trying to upset it is is a little more than compensated for by the bullet spinning faster at higher velocity from a particular twist barrel.
Bullet shape, length, and and mass distribution inside the bullet all matter. A given twist barrel can stabilize a much heavier tungsten core bullet than a copper alloy solid. For the same weight the copper alloy bullet will be much longer. For copper jacketed lead typically flat base bullets are shorter and more stable than VLD designs of the same weight.
As others have stated the only way to know for sure is to shoot some. Keep in mind that a bullet which shoots great in hot weather or high altitude could be unstable in cold weather or lower altitude using the same rifle and loads.
Higher humidity being less dense may seem counter intuitive, but H20 molecules are lighter than the other molecules which make up air which are N2 O2, CO2 and Ar.