The current Howa .223 is 1-12" twist. I just bought my wife one for Xmas. It will indeed handle 50gr bullets nicely, as the .681" and .518" groups my wife shot last weekend will atest to. The Howa is indeed a fine rifle, fit and finish is great, and my wife's gun cleans up just as easy as my $300 PacNor SS match barrel.
Now if a 1-12" twist is gonna be finicky with 40gr bullets in a .223, why did my old .223 with a 1-15" twist Hart barrel shoot 50gr bullets of several types and Mfg into groups consistently under 1/2" and some into the .230" range? And before ya start spouting about long range instability, I killed lots of prairie dogs with that gun at over 500yds. My dad also has a .223 with a 1-14" twist that shoots as good or better than mine did with 50gr bullets, and has killed even more prairie dogs at 500+ yds.
How does that figure into your calculations?