Win a Duel in Armadillo – Travel to the town of Armadillo, and accept the challenge to fight a duel. Just make sure to win.

Capture or Kill Mo Van Barr – While in Armadillo, find the wanted poster for Mo Van Barr, accept the Bounty Hunting mission and bring this notorious member of the Walton Gang to justice, dead or alive.

Solve the Mystery of the Missing Townsfolk – People have been disappearing for weeks from the outskirts of Armadillo. Track down the clues, solve the mystery and live to tell the tale.

Search Coot's Chapel – Find the ruins of Coot's chapel in the wilds of Cholla Springs, and search it for treasure.

Complete the Twin Rocks Hideout – Find the legendary criminal hideout of Twin Rocks, located North of the town of Armadillo, to complete this final challenge.

Kotaku? One of my least favourite sites. 
 
Anywho, thanks for passing on the tip.

I've already got the Mo Van Barr poster, gonna do that after I get off work.  I also already took care of the Twin Rocks Hideout, and I've been to Coot's Chapel but haven't searched around it.  I need a duel in Armadillo, though...and that damn quest for finding all the missing people is taking its sweet goddamn time.