I haven't used the 556, but I have used the 55 & 75 grain 223 ammo and it's been very accurate. I've shot the 75 grain stuff out to 500 yards in my savage with a 9 twist barrel, it would be worth a try to see if it stabilizes in yours. But as for the steel vs brass case, I've had no issues with it, and it shoots as well as the brass cased match ammo.