What headspace guage for homebuilt AR-15

What headspace gauge should I use on a homebuilt AR-15? I am going to go with a 5.56 barrel but I will also be using .223 in it.

I am ordering a BUNCH of stuff from brownells so thats where I plan to get the gauge as well. I am looking at the following.

5.56mm (.223) HEADSPACE GAUGE
http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=6646&title=5.56mm+(.223)+HEADSPACE+GAUGE

MATCH RIFLE HEADSPACE GAUGES
http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=6635&title=MATCH+RIFLE+HEADSPACE+GAUGES

FORSTER
http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=19183&title=.17+-+.22+CALIBER+RIFLE+HEADSPACE+GAUGES

CLYMER
http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=19198&title=.17+-+.22+CAL.+RIFLE+HEADSPACE+GAUGES