You can find .223 BCG Bolt Carrier Groups in many different types, calibers, and styles at the Daytona Tactical online store.  Most people will say that the M16 style is the best choice since it is full-auto capable (not by itself though) and more reliable.  The .223 BCG M16 style is the most reliable especially if it is a duty weapon or the shooter’s only AR.  AR-15 style .223 BCG are an acceptable alternative if weight is a concern though.