Re: 14 twist barrel and 68 gr .224 bullets
My experience is that a 1 in 10 twist is needed to stabilise the 68 and 69 grain bullets.
Just after the 68 Hornady and 69 Sierra became available in Oz in 1986, I built up a custom 220 Swift AI, with a 1 in 12 twist barrel (gunsmiths suggestion). However, quickly found that despite reaching about 3650 fps with these bullets, they keyholed into the target at 100 yards.
About 2 years later built up a .224 Clark with 1 in 10 twist, and the 69 grain Sierra grouped well at velocities of 3850 - 3900 fps. About 2 years ago, and after about 1600 shots, I had that barrel set back, and rechambered to the 22/250 AI, and when using the 69 grain Sierra, it consistently groups between .25 and .5 moa with a variety of powders at velocities of 3350 - 3430 fps.
About 35 years ago, I owned a custom 22/250 with a 1 in 14 twist barrel, and it liked and shot best with the Sierra 53 grain HP (3700+ fps). It gave reasonable accuracy (.75moa) with the Hornady 60 grain soft point, but it definitely seemed to prefer bullets in the 50 - 55 grain range. Brian.