Rob,
It all kinda depends on how far you want to be shooting, whether or not you want to have a front row seat on the explosion, and how much money you have to spend.
I do a lot of dogging with my BM, and I have a Weaver 6.5-20 on mine. If you plan on spending many hours behind the glass, and are on a budget, I would suggest a little less magnification, and a little better glass. My Weaver is O.K., but not great. 9 power is not bad, unless you plan on shooting mostly over a couple of hundred yards. I would highly recommend that you look through the glass before buying. And I mean look at some distant objects, outside if possible, not just the corners of the shop. If you have room, set a pop can out a few hundred yards, and then you can decide if the picture is big enough. You will get a different opinion from everyone on glass, but one good generality is that you do get what you pay for. A poorly made scope with high magnification will leave you cross-eyed, in no time flat.
Luckily the 223 is not hard on scopes, so you do not need one built like a tank. As a minimum, I would try to find one with coated lenses, and a decent field of few. Like the other poster mentioned, a fine cross hair is very helpful. I like my target knobs, for getting way out there.
Since you did not mention a budget, it is hard to recommend any particular brand, but a little web hunting in the Varmint Circles should help a lot.
Craig
VHA Member