Vihtavuori N-133 works extremely well in 223 in both bolt guns and ARs. Try ~23.7 grains as a flexible, accurate charge weight behind 55-grain bullets (working up from below to assure safety in your rifle, of course). Sierra lists an N-133 load as the accuracy load for 55-grain bullets at 24.2 or 24.3 grains (can't recall which). Again, work up to that.
Winchester 748 is also excellent in 223, and is flexible up to higher bullet weights. ~26.6 grains (working up from below for safety) is a proven flexible and accurate load.
Finally, it is my understanding that the AR-15 was designed by Eugene Stoner around IMR4198 powder.