I am looking at casting 45 gr. Lymans for my .223. In my reading/research (see Castpics articles on .223 cast bullets) I've found that the bullet casting isn't a problem, it's driving the lead slug fast enough to cycle the AR action. From what I remember, the velocities/pressures needed (2200 fps+) are difficult to reach successfully (leading, accuracy, etc.)...,
But, I could be wrong, I don't have an AR and I only shoot my .223 one at a time