GWC - Like BB said, I would get a combo set (.223/20Ga), but I don't believe they make it, in that combo caliber/Ga.. If they don't or you can't get one, purchase a .223 rifle (SB2 frame) and send it in for a 20 Ga barrel with screw in chokes, and a .50Cal Huntsman muzzle loader barrel. This is the easiest way to get what you want.
The entire set up will only work with an SB2 frame.
If you want to change the stock (from a Monte Carlo to straight), just ask H&R to also include a stock.
If you want to spend a little less money, and some extra time hunting for the extra barrels on this site, have fun with the classifieds.
In the interest of fair play I have to warn you. Your browsing in the classifieds might cost you more that what you originally intended to spend, since you'll be tempted by all the goodies.
Best of luck.