Its my opinion that anything based off the 223 case is perfect for the contender design. I like the 7 TCU best out of all the chamberings I have owned or seen. I sold all others and have one 10 inch and one 14 inch 7TCU barrels.
With larger case head sizes some people index the round by marking the back of the head and putting that up top on each loading of a round. This is due to the angle of the head after firing. The 223 based rounds do not need this even though some folks still do it.