I found a mint used Winchester Lightweight Model 70 in .223 and have taken a lot of groundhogs with it. I would look at the used gun racks and see what is available, handles well and has a decent price. The link below came from another site and states that the 5.56mm and .223 are not the same. It is possible to shoot .223 ammo in a 5.56mm. However, there are warnings about shooting 5.56mm in a .223 caliber rifle. I have a pair of Colt ARs that are marked .223 on the receiver and 5.56mm on the barrel so they can handle both rounds. Hope this helps. All the best...
Gil
http://www.thegunzone.com/556v223.html