If your Colt is An AR type rifle 77 and 75 grain bullets usually require a 1/7 or1/8 twist to stabalize them but not always a 1/9 will work for the 75 in some cases. The shootersI know use Reloader 15 or Varget powder for there 223 service rifle ammo and don't change . Hope this helps, and as always this is just my opinion.