Barrel twist rate for a 223 Rem?

If you ever plan on shooting heavier than 55 gr bullets, I would get the 1:8. Be warned if you shoot some lighter bullets, they may disintegrate shortly after leaving the barrel and you may have to select a different bullet. My son and I just built an upper for his AR-15. It has a 20" barrel with a 1:7 twist rate. It will shoot hand loaded Nosler Ballistic Tip 55 gr bullets and make touching holes at 100 yds. Those are the only bullets that we have tried with it, but he wanted the capability to shoot heavy bullets at longer distances.