Re: Question on 223 rem. reload
It's hard to say what would be best in Your rifle. You should test them both and see what your rifle likes.
4320 is a smaller cut and will be easier to load into the small 223 case mouth without bridging, but it will also get you about 100fps less at the same pressures as 3031. I guess it depends on what you want and how you plan on loading.
I meter my 223/5.56 loads and prefer "ball" type powders like Tac, 2230/X-T, 748 and 335.