To my limited knowlege, Frontier is a plastic lower receiver instead of aluminum unless they're now offering both.
And you can shoot 223 in a 5.56 AR. .223 is the commercial version. Slight case and powder differences, but compatible with either nonetheless.
5.56 is a little hotter.
5.56 in a 223 chamber is bad, but 223 in a 556 chamber is alright.
I don't know of any 15 round count AR mags, but there are lots of 5 and 10 rounders.
if you give them an inch, eventually they will be a ruler.