Lone Ranger Gatling Gun Question

[hypergirl31]

I was playing the Lone Ranger play set a little after I just beat the game, and I am trying to buy the Gatling gun. I saw on the internet that I would have to complete A Heap A Trouble mission before getting it. But I can't seem to find this mission. Therefore, I can't do the Training your Sights mission. Can someone please help? Thanks!